Java Generics Tutorial: Classes, Methods & Wildcards

Introduction

Java Generics allow you to write classes, interfaces, and methods that work with different data types while maintaining compile-time type safety.

Before Java 5, collections commonly stored objects as the Object type. Developers then had to manually cast values back to their expected type.

For example:

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1import java.util.ArrayList;
2
3public class Main {
4
5    public static void main(String[] args) {
6
7        ArrayList list = new ArrayList();
8
9        list.add("Java");
10        list.add(100);
11
12        String language = (String) list.get(1);
13
14        System.out.println(language);
15    }
16}

The code compiles, but it fails at runtime because the second element is an Integer, not a String.

textCopy
1Exception in thread "main" java.lang.ClassCastException

Generics solve this problem by allowing you to specify the type of data a collection or class is expected to contain.

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1ArrayList<String> languages = new ArrayList<>();
2
3languages.add("Java");
4languages.add("Python");

Now the compiler prevents incompatible values from being added:

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1languages.add(100); // Compile-time error

This is one of the biggest advantages of generics: errors are detected during compilation instead of appearing unexpectedly at runtime.

Generics are extensively used in:

• Java Collections Framework
• Java Streams
• Spring Boot
• Hibernate
• Java APIs
• Enterprise applications
• Generic repositories
• Data-access layers
• Utility classes

In this tutorial, you will learn Java Generics from the fundamentals through practical examples.

Table of Contents

1. What Are Generics?
2. Why Use Generics?
3. Generic Classes
4. Multiple Type Parameters
5. Generic Methods
6. Generic Constructors
7. Generic Interfaces
8. Unbounded Wildcards
9. Upper Bounded Wildcards
10. Lower Bounded Wildcards
11. Bounded Type Parameters
12. Multiple Bounds
13. Type Erasure
14. Generics vs Object
15. Generic Naming Conventions
16. Real-World Uses of Generics
17. Best Practices
18. Practical Generic Project
19. Practice Exercises
20. Summary
21. SEO Keywords

What Are Generics?

Generics are a Java language feature that allows you to define code using type parameters.

Instead of hard-coding one particular data type, a generic class or method can work with different types.

For example:

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1class Box<T> {
2
3    private T value;
4
5}

Here, T is a type parameter.

When the class is used, T can represent a specific reference type:

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1Box<String>
2Box<Integer>
3Box<Double>

The same class can therefore be reused with different types.

Generic Syntax

A generic class typically looks like this:

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1class ClassName<T> {
2
3    // class members
4
5}

Common type parameters include:

These are conventions rather than mandatory names.

You could technically write:

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1class Box<MyType> {
2}

However, following standard Java naming conventions makes generic code easier for other developers to understand.

Why Use Generics?

Generics provide several important benefits.

Type Safety

Generics prevent incompatible types from being inserted into generic structures.

javaCopy
1import java.util.ArrayList;
2import java.util.List;
3
4List<String> languages = new ArrayList<>();
5
6languages.add("Java");
7languages.add("Python");
8
9// languages.add(100); // Compile-time error

The compiler knows that the list should contain only String values.

Compile-Time Error Checking

Without generics, some problems are detected only when the application runs.

With generics, many type-related errors are detected while compiling the application.

This makes applications safer and easier to maintain.

No Unnecessary Casting

Without generics:

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1Object value = "Java";
2
3String language = (String) value;

With generics:

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1Box<String> box = new Box<>();
2
3String language = box.getValue();

The compiler already knows that getValue() returns a String.

Code Reusability

A generic class can work with multiple types instead of requiring separate implementations.

For example:

javaCopy
1Box<String>
2Box<Integer>
3Box<Double>

can all use the same Box<T> implementation.

Better API Design

Generics make method parameters and return types clearer.

For example:

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1public static <T> T getFirst(List<T> items) {
2    return items.get(0);
3}

The relationship between the input type and return type is clear.

Generic Classes

A generic class contains one or more type parameters.

Basic Generic Class

javaCopy
1class Box<T> {
2
3    private T value;
4
5    public void setValue(T value) {
6        this.value = value;
7    }
8
9    public T getValue() {
10        return value;
11    }
12}

The class does not decide what T means.

The caller determines the type when creating the object.

Using a Generic Class

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1public class Main {
2
3    public static void main(String[] args) {
4
5        Box<String> languageBox = new Box<>();
6
7        languageBox.setValue("Java");
8
9        String language = languageBox.getValue();
10
11        System.out.println(language);
12
13        Box<Integer> numberBox = new Box<>();
14
15        numberBox.setValue(100);
16
17        Integer number = numberBox.getValue();
18
19        System.out.println(number);
20    }
21}

Output:

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1Java
2100

How It Works

When you write:

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1Box<String> languageBox = new Box<>();

T becomes String for that particular object.

Therefore:

javaCopy
1setValue(T value)

effectively expects:

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1setValue(String value)

and:

javaCopy
1getValue()

returns:

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1String

For:

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1Box<Integer> numberBox = new Box<>();

T becomes Integer.

Multiple Type Parameters

A generic class can define multiple type parameters.

A common example is a key-value pair.

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1class Pair<K, V> {
2
3    private final K key;
4    private final V value;
5
6    public Pair(K key, V value) {
7        this.key = key;
8        this.value = value;
9    }
10
11    public K getKey() {
12        return key;
13    }
14
15    public V getValue() {
16        return value;
17    }
18
19    public void display() {
20        System.out.println(key + " : " + value);
21    }
22}

Using Pair

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1public class Main {
2
3    public static void main(String[] args) {
4
5        Pair<Integer, String> student =
6                new Pair<>(101, "Ankit");
7
8        System.out.println(
9                "ID: " + student.getKey()
10        );
11
12        System.out.println(
13                "Name: " + student.getValue()
14        );
15    }
16}

Output:

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1ID: 101
2Name: Ankit

Here:

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1Pair<Integer, String>

means:

• K is Integer
• V is String

The same class can be reused:

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1Pair<String, Double> product =
2        new Pair<>("Laptop", 79999.0);

Generic Methods

A generic method defines its own type parameter.

The generic type parameter is placed before the return type.

Syntax

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1public static <T> T methodName(T value) {
2
3    return value;
4}

Here, <T> declares the method's type parameter.

Example

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1public class Utility {
2
3    public static <T> void printValue(T value) {
4        System.out.println(value);
5    }
6
7    public static <T> T getValue(T value) {
8        return value;
9    }
10}

Using the methods:

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1public class Main {
2
3    public static void main(String[] args) {
4
5        Utility.printValue("Java");
6        Utility.printValue(100);
7        Utility.printValue(99.99);
8
9        String language =
10                Utility.getValue("Java");
11
12        Integer number =
13                Utility.getValue(500);
14
15        System.out.println(language);
16        System.out.println(number);
17    }
18}

Output:

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1Java
2100
399.99
4Java
5500

Generic Method with a List

Generic methods become particularly useful when working with collections.

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1import java.util.List;
2
3public class ListUtils {
4
5    public static <T> T getFirst(List<T> items) {
6
7        if (items == null || items.isEmpty()) {
8            throw new IllegalArgumentException(
9                    "List must not be null or empty."
10            );
11        }
12
13        return items.get(0);
14    }
15}

Usage:

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1import java.util.List;
2
3public class Main {
4
5    public static void main(String[] args) {
6
7        List<String> languages =
8                List.of("Java", "Python", "C++");
9
10        String first =
11                ListUtils.getFirst(languages);
12
13        System.out.println(first);
14    }
15}

Output:

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1Java

The compiler automatically infers the type parameter.

Generic Constructors

A constructor can also declare its own generic type parameter.

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1class Display {
2
3    public <T> Display(T value) {
4        System.out.println(value);
5    }
6}

Usage:

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1public class Main {
2
3    public static void main(String[] args) {
4
5        new Display("Java");
6        new Display(100);
7        new Display(99.5);
8    }
9}

Output:

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1Java
2100
399.5

Notice that the class itself is not generic.

The constructor independently declares:

javaCopy
1<T>

Generic Interfaces

Interfaces can also use generic type parameters.

For example:

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1interface Repository<T> {
2
3    void save(T item);
4
5    T findById(int id);
6}

A concrete implementation can specify the type.

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1class StudentRepository
2        implements Repository<Student> {
3
4    @Override
5    public void save(Student student) {
6        System.out.println(
7                "Saving: " + student.getName()
8        );
9    }
10
11    @Override
12    public Student findById(int id) {
13        return new Student(id, "Ankit");
14    }
15}

The Student class:

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1class Student {
2
3    private final int id;
4    private final String name;
5
6    public Student(int id, String name) {
7        this.id = id;
8        this.name = name;
9    }
10
11    public int getId() {
12        return id;
13    }
14
15    public String getName() {
16        return name;
17    }
18}

Now the repository is type-safe:

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1StudentRepository repository =
2        new StudentRepository();
3
4repository.save(
5        new Student(101, "Ankit")
6);

This pattern is common in application architecture, especially in data-access layers.

Wildcards

Sometimes a method does not need to know the exact generic type.

Java provides the wildcard:

javaCopy
1?

A wildcard represents an unknown type.

For example:

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1List<?>

means a list containing some unknown type.

Unbounded Wildcard

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1import java.util.List;
2
3public class ListPrinter {
4
5    public static void printList(List<?> items) {
6
7        for (Object item : items) {
8            System.out.println(item);
9        }
10    }
11}

This method can accept lists of different types:

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1import java.util.List;
2
3public class Main {
4
5    public static void main(String[] args) {
6
7        List<String> names =
8                List.of("Ankit", "Rahul");
9
10        List<Integer> numbers =
11                List.of(10, 20, 30);
12
13        ListPrinter.printList(names);
14        ListPrinter.printList(numbers);
15    }
16}

Output:

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1Ankit
2Rahul
310
420
530

Why Can We Read as Object?

Because every reference type in Java ultimately derives from Object, values read from List<?> can safely be assigned to Object.

However, you generally cannot add arbitrary values to a List<?> because the actual type is unknown.

Upper Bounded Wildcards

An upper bounded wildcard uses:

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1? extends Type

For example:

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1List<? extends Number>

means the list contains some type that extends Number.

It could be:

javaCopy
1List<Integer>
2List<Double>
3List<Float>

Example

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1import java.util.List;
2
3public class Calculator {
4
5    public static double sum(
6            List<? extends Number> numbers) {
7
8        double total = 0.0;
9
10        for (Number number : numbers) {
11            total += number.doubleValue();
12        }
13
14        return total;
15    }
16}

Usage:

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1import java.util.List;
2
3public class Main {
4
5    public static void main(String[] args) {
6
7        List<Integer> integers =
8                List.of(10, 20, 30);
9
10        List<Double> decimals =
11                List.of(1.5, 2.5, 3.5);
12
13        System.out.println(
14                Calculator.sum(integers)
15        );
16
17        System.out.println(
18                Calculator.sum(decimals)
19        );
20    }
21}

Output:

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160.0
27.5

The method can read values as Number regardless of the specific numeric subtype.

Lower Bounded Wildcards

A lower bounded wildcard uses:

javaCopy
1? super Type

For example:

javaCopy
1List<? super Integer>

means the list can be a list of Integer, Number, or Object.

Example

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1import java.util.List;
2
3public class NumberUtils {
4
5    public static void addNumbers(
6            List<? super Integer> numbers) {
7
8        numbers.add(10);
9        numbers.add(20);
10        numbers.add(30);
11    }
12}

The method can accept:

javaCopy
1List<Integer>
2List<Number>
3List<Object>

Example:

javaCopy
1import java.util.ArrayList;
2import java.util.List;
3
4public class Main {
5
6    public static void main(String[] args) {
7
8        List<Integer> integers =
9                new ArrayList<>();
10
11        List<Number> numbers =
12                new ArrayList<>();
13
14        List<Object> objects =
15                new ArrayList<>();
16
17        NumberUtils.addNumbers(integers);
18        NumberUtils.addNumbers(numbers);
19        NumberUtils.addNumbers(objects);
20
21        System.out.println(integers);
22        System.out.println(numbers);
23        System.out.println(objects);
24    }
25}

Output:

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1[10, 20, 30]
2[10, 20, 30]
3[10, 20, 30]

PECS: Producer Extends, Consumer Super

A useful rule for remembering wildcard bounds is PECS:

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1Producer → extends
2Consumer → super

If a collection produces values for your code to read:

javaCopy
1List<? extends Number>

If a collection consumes values that your code wants to add:

javaCopy
1List<? super Integer>

This principle is particularly useful when designing flexible generic methods.

Bounded Type Parameters

Sometimes a generic type should be restricted to a particular class hierarchy.

A bounded type parameter uses:

javaCopy
1<T extends Type>

For example:

javaCopy
1<T extends Number>

means T must be Number or a subclass of Number.

Example

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1class Calculator<T extends Number> {
2
3    private final T number;
4
5    public Calculator(T number) {
6        this.number = number;
7    }
8
9    public double square() {
10
11        double value = number.doubleValue();
12
13        return value * value;
14    }
15}

Usage:

javaCopy
1public class Main {
2
3    public static void main(String[] args) {
4
5        Calculator<Integer> integerCalculator =
6                new Calculator<>(5);
7
8        Calculator<Double> doubleCalculator =
9                new Calculator<>(5.5);
10
11        System.out.println(
12                integerCalculator.square()
13        );
14
15        System.out.println(
16                doubleCalculator.square()
17        );
18    }
19}

Output:

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125.0
230.25

The bound gives the compiler permission to use methods available on Number, such as:

javaCopy
1doubleValue()
2intValue()
3longValue()
4floatValue()

Multiple Bounds

A generic type parameter can have multiple bounds.

For example:

javaCopy
1<T extends Number & Comparable<T>>

The first bound must be a class, while additional bounds can be interfaces.

Example:

javaCopy
1public static <T extends Number & Comparable<T>>
2T maximum(T first, T second) {
3
4    return first.compareTo(second) >= 0
5            ? first
6            : second;
7}

Usage:

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1Integer result =
2        maximum(20, 10);
3
4System.out.println(result);

Output:

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120

Multiple bounds allow generic code to require multiple capabilities from a type.

Type Erasure

Java implements generics primarily through type erasure.

Generic type information is used by the compiler for type checking, but generic type arguments are generally not retained as runtime type information in the same way as ordinary class metadata.

For example:

javaCopy
1List<String>
2List<Integer>

are different generic types at compile time, but runtime type information does not distinguish these type arguments in the same way.

Type erasure has several important consequences.

Generic Arrays

You cannot directly create:

javaCopy
1new T[10]

inside a generic class.

For example:

javaCopy
1class Storage<T> {
2
3    // T[] items = new T[10]; // Not allowed
4}

Primitive Type Arguments

Generics work with reference types, not primitive types.

This is invalid:

javaCopy
1List<int> numbers;

Use the wrapper type instead:

javaCopy
1List<Integer> numbers;

Java's autoboxing and unboxing make working with wrapper types convenient.

Runtime Type Checks

Because generic type arguments are erased, code such as:

javaCopy
1if (value instanceof List<String>) {
2    // ...
3}

is not allowed.

You can check the raw generic class:

javaCopy
1if (value instanceof List<?>) {
2    // ...
3}

but not the specific type argument at runtime.

Generics vs Object

Consider a collection without generics:

javaCopy
1import java.util.ArrayList;
2
3ArrayList list = new ArrayList();
4
5list.add("Java");
6
7String value = (String) list.get(0);

The developer has to perform an explicit cast.

With generics:

javaCopy
1import java.util.ArrayList;
2import java.util.List;
3
4List<String> list = new ArrayList<>();
5
6list.add("Java");
7
8String value = list.get(0);

The compiler already knows that the list contains String values.

Comparison

Real-World Uses of Generics

Generics are everywhere in modern Java development.

Collections

javaCopy
1List<String> names;
2
3Set<Integer> numbers;
4
5Map<Integer, String> students;

Optional

javaCopy
1Optional<String> name;

Comparable

javaCopy
1class Student
2        implements Comparable<Student> {
3
4    @Override
5    public int compareTo(Student other) {
6        return 0;
7    }
8}

Comparator

javaCopy
1Comparator<Student> comparator;

Generic Repositories

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1interface Repository<T> {
2
3    void save(T entity);
4
5    T findById(long id);
6}

Generic API Responses

Backend applications frequently define reusable response wrappers:

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1class ApiResponse<T> {
2
3    private final T data;
4    private final String message;
5
6    public ApiResponse(T data, String message) {
7        this.data = data;
8        this.message = message;
9    }
10
11    public T getData() {
12        return data;
13    }
14
15    public String getMessage() {
16        return message;
17    }
18}

Now different API responses can use the same structure:

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1ApiResponse<String> messageResponse;
2
3ApiResponse<Student> studentResponse;
4
5ApiResponse<List<Student>> studentsResponse;

This is one reason generics are so important in backend Java development.

Generic Utility Example

A practical generic utility can return the last element of a list.

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1import java.util.List;
2
3public final class CollectionUtils {
4
5    private CollectionUtils() {
6        // Prevent object creation.
7    }
8
9    public static <T> T getLast(List<T> items) {
10
11        if (items == null || items.isEmpty()) {
12            throw new IllegalArgumentException(
13                    "List must not be null or empty."
14            );
15        }
16
17        return items.get(items.size() - 1);
18    }
19}

Usage:

javaCopy
1import java.util.List;
2
3public class Main {
4
5    public static void main(String[] args) {
6
7        List<String> languages =
8                List.of("Java", "Python", "C++");
9
10        List<Integer> numbers =
11                List.of(10, 20, 30);
12
13        String language =
14                CollectionUtils.getLast(languages);
15
16        Integer number =
17                CollectionUtils.getLast(numbers);
18
19        System.out.println(language);
20        System.out.println(number);
21    }
22}

Output:

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1C++
230

The same method works for different types without duplicating the implementation.

Best Practices

Use Generics with Collections

Prefer:

javaCopy
1List<String> names = new ArrayList<>();

instead of:

javaCopy
1List names = new ArrayList();

Raw types remove much of the compile-time type safety provided by generics.

Prefer the Diamond Operator

Instead of:

javaCopy
1List<String> names =
2        new ArrayList<String>();

prefer:

javaCopy
1List<String> names =
2        new ArrayList<>();

The compiler can infer the type argument.

Use Appropriate Bounds

If a method only needs to read numeric values, an upper bound can make the API more flexible:

javaCopy
1List<? extends Number>

If a method needs to add Integer values:

javaCopy
1List<? super Integer>

Keep Generic APIs Simple

Do not introduce complex generic parameters unless they provide a real benefit.

Good generic code should make an API more reusable without making it unnecessarily difficult to understand.

Avoid Raw Types

Avoid:

javaCopy
1List list;
2Map map;

Prefer:

javaCopy
1List<String> names;
2Map<Integer, String> students;

Use Meaningful Type Parameters

Use conventional names when they communicate the purpose clearly:

javaCopy
1<T>
2<E>
3<K, V>

For more complex APIs, descriptive names can sometimes improve readability.

Practice Project: Generic Storage

Let's build a reusable generic storage class.

javaCopy
1class Storage<T> {
2
3    private T item;
4
5    public void store(T item) {
6        this.item = item;
7    }
8
9    public T retrieve() {
10        return item;
11    }
12
13    public boolean isEmpty() {
14        return item == null;
15    }
16}

Using the Generic Storage

javaCopy
1public class Main {
2
3    public static void main(String[] args) {
4
5        Storage<String> languageStorage =
6                new Storage<>();
7
8        languageStorage.store("Java");
9
10        System.out.println(
11                languageStorage.retrieve()
12        );
13
14        Storage<Integer> numberStorage =
15                new Storage<>();
16
17        numberStorage.store(500);
18
19        System.out.println(
20                numberStorage.retrieve()
21        );
22
23        System.out.println(
24                "Storage empty: "
25                        + numberStorage.isEmpty()
26        );
27    }
28}

Output:

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1Java
2500
3Storage empty: false

The same Storage<T> class can store different reference types.

Practice Project: Generic Repository

Create a generic repository interface:

javaCopy
1interface Repository<T> {
2
3    void save(T item);
4
5    T findById(int id);
6
7    void delete(T item);
8}

Create a student model:

javaCopy
1class Student {
2
3    private final int id;
4    private final String name;
5
6    public Student(int id, String name) {
7        this.id = id;
8        this.name = name;
9    }
10
11    public int getId() {
12        return id;
13    }
14
15    public String getName() {
16        return name;
17    }
18
19    @Override
20    public String toString() {
21        return "Student{id=" + id
22                + ", name='" + name + "'}";
23    }
24}

A simple implementation can use a list:

javaCopy
1import java.util.ArrayList;
2import java.util.List;
3
4class StudentRepository
5        implements Repository<Student> {
6
7    private final List<Student> students =
8            new ArrayList<>();
9
10    @Override
11    public void save(Student student) {
12        students.add(student);
13    }
14
15    @Override
16    public Student findById(int id) {
17
18        for (Student student : students) {
19
20            if (student.getId() == id) {
21                return student;
22            }
23        }
24
25        return null;
26    }
27
28    @Override
29    public void delete(Student student) {
30        students.remove(student);
31    }
32}

Usage:

javaCopy
1public class Main {
2
3    public static void main(String[] args) {
4
5        StudentRepository repository =
6                new StudentRepository();
7
8        repository.save(
9                new Student(101, "Ankit")
10        );
11
12        repository.save(
13                new Student(102, "Rahul")
14        );
15
16        Student student =
17                repository.findById(101);
18
19        System.out.println(student);
20    }
21}

Output:

textCopy
1Student{id=101, name='Ankit'}

This project demonstrates how generics can be used to create reusable type-safe application components.

Practice Exercises

Exercise 1: Generic Stack

Create a generic Stack<T> class with:

• push(T item)
• pop()
• peek()
• isEmpty()
• size()

Test it with both String and Integer.

Exercise 2: Generic Pair

Create a Pair<K, V> class that stores:

• A key
• A value

Add methods to:

• Get the key
• Get the value
• Update the value
• Display both values

Exercise 3: Generic Calculator

Create:

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1Calculator<T extends Number>

Add methods to calculate:

• Square
• Cube
• Double value
• Integer value

Test the class with Integer and Double.

Exercise 4: Generic Repository

Create:

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1Repository<T>

with methods:

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1save(T item)
2findById(int id)
3delete(T item)

Implement the repository for a Student class.

Exercise 5: Generic Utility Methods

Create a utility class with generic methods for:

• Finding the first element
• Finding the last element
• Swapping two elements
• Printing a list
• Checking whether a list is empty

Exercise 6: Student Management System

Build a small student management application that:

• Stores students using List<Student>
• Uses a generic Repository<Student>
• Searches students by ID
• Displays all students
• Deletes students
• Uses generic utility methods
• Demonstrates bounded types where appropriate

Common Mistakes with Generics

Using Raw Collections

Avoid:

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1List students = new ArrayList();

Prefer:

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1List<Student> students = new ArrayList<>();

Confusing extends with super

Remember:

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1? extends T

is generally useful when consuming values from a producer.

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1? super T

is generally useful when supplying values to a consumer.

The PECS rule helps remember this:

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1Producer Extends
2Consumer Super

Using Primitive Types

This is invalid:

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1List<int> numbers;

Use:

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1List<Integer> numbers;

Overcomplicating Generic APIs

Generics should improve reusability and type safety. If a generic design becomes unnecessarily complicated, simplify it.

Summary

In this Java Generics tutorial, you learned how generics make Java programs more reusable, readable, and type-safe.

You learned:

• What Java Generics are
• Why generics are useful
• How generic classes work
• How to create classes with multiple type parameters
• How to create generic methods
• How generic constructors work
• How to create generic interfaces
• How unbounded wildcards work
• How ? extends works
• How ? super works
• How bounded type parameters work
• How multiple bounds work
• What type erasure means
• Why generic arrays and primitive type arguments have restrictions
• The difference between generics and raw Object types
• Common generic naming conventions
• How generics are used in collections and backend applications
• How to create reusable generic utilities
• How to build a generic repository

Generics are a fundamental part of modern Java programming. You will encounter them frequently when working with collections, streams, repositories, APIs, Spring Boot, Hibernate, and other Java frameworks.

A strong understanding of generics will also make advanced Java topics such as Collections, Lambda Expressions, Streams, Functional Programming, and Spring Boot easier to understand.

Practice Challenge

Build a complete generic data-management system.

Your application should contain:

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1GenericRepository<T>
2        |
3        +── save()
4        +── findById()
5        +── findAll()
6        +── delete()

Then implement it for:

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1Student
2Employee
3Product

The goal is to use one generic repository design for multiple domain models while maintaining compile-time type safety.