Generic Methods and Bounded Type Parameters (<T extends Number>)0%

Wildcards in Generics: Upper Bounded (? extends) and Lower Bounded (? super)

Generic Methods and Bounded Type Parameters (<T extends Number>)

Advanced15 min read•Updated: 2025-01-15

⚡ Generic Methods and Bounded Type Parameters (<T extends Number>)

Introduction

While generic classes parameterize an entire class, generic methods introduce their own type parameters scoped solely to that specific method. Furthermore, when you need to restrict the allowable types to subclasses of a specific class or implementors of an interface, Java provides Bounded Type Parameters using the extends keyword.

🛠️ 1. Generic Methods Syntax

A generic method declares its type parameter(s) before the method's return type:

1public class ArrayUtilities {
2 
  // <T> before the return type marks this as a generic method
  public static <T> void swap(T[] array, int i, int j) {
      T temp = array[i];
      array[i] = array[j];
      array[j] = temp;
  }
9 
  public static <T> void printArray(T[] array) {
      for (T element : array) {
          System.out.print(element + " ");
      }
      System.out.println();
  }
16}

🎯 2. Bounded Type Parameters (`<T extends SuperClass>`)

Sometimes an algorithm requires knowledge about the capabilities of T. For example, calculating numeric statistics requires mathematical operations, which are available on subclasses of java.lang.Number.

1package com.mskinstitute.generics;
2 
3public class StatsCalculator<T extends Number> {
4    private final T[] numbers;
5 
6    public StatsCalculator(T[] numbers) {
7        this.numbers = numbers;
8    }
9 
10    public double calculateAverage() {
11        double sum = 0.0;
12        for (T num : numbers) {
13            // Because T extends Number, we can safely call doubleValue()!
14            sum += num.doubleValue();
15        }
16        return sum / numbers.length;
17    }
18}

Valid vs Invalid Usage:

1// ✅ Valid: Integer and Double extend java.lang.Number
2StatsCalculator<Integer> intStats = new StatsCalculator<>(new Integer[]{10, 20, 30});
3StatsCalculator<Double> doubleStats = new StatsCalculator<>(new Double[]{98.5, 87.2, 91.0});
4 
5// ❌ COMPILE ERROR: String does NOT extend Number!
6// StatsCalculator<String> strStats = new StatsCalculator<>(new String[]{"A", "B"});

🔗 3. Multiple Bounds (`<T extends Class & Interface>`)

In Java, a type parameter can have multiple bounds. If one of the bounds is a class, it must be listed first, followed by interface bounds separated by &:

1// T must be a Number AND must implement Comparable<T>
2public static <T extends Number & Comparable<T>> T findMax(T a, T b) {
3    if (a.compareTo(b) >= 0) {
4        return a;
5    }
6    return b;
7}

⚠️ Common Pitfalls

1
Primitive Types as Type Arguments: You cannot use primitives like int or double directly in generics (StatsCalculator<int> ❌). You must use wrapper classes (StatsCalculator<Integer> ✅).
2
Order of Bounds: Class bounds must always precede interface bounds: <T extends Serializable & Number> ❌ compile error; <T extends Number & Serializable> ✅ correct.

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Wildcards in Generics: Upper Bounded (? extends) and Lower Bounded (? super)

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Wildcards in Generics: Upper Bounded (? extends) and Lower Bounded (? super)

Generic Methods and Bounded Type Parameters (<T extends Number>)

Advanced15 min read•Updated: 2025-01-15

Study Materials

⚡ Generic Methods and Bounded Type Parameters (<T extends Number>)

Introduction

🛠️ 1. Generic Methods Syntax

A generic method declares its type parameter(s) before the method's return type:

1public class ArrayUtilities {
2 
  // <T> before the return type marks this as a generic method
  public static <T> void swap(T[] array, int i, int j) {
      T temp = array[i];
      array[i] = array[j];
      array[j] = temp;
  }
9 
  public static <T> void printArray(T[] array) {
      for (T element : array) {
          System.out.print(element + " ");
      }
      System.out.println();
  }
16}

🎯 2. Bounded Type Parameters (`<T extends SuperClass>`)

1package com.mskinstitute.generics;
2 
3public class StatsCalculator<T extends Number> {
4    private final T[] numbers;
5 
6    public StatsCalculator(T[] numbers) {
7        this.numbers = numbers;
8    }
9 
10    public double calculateAverage() {
11        double sum = 0.0;
12        for (T num : numbers) {
13            // Because T extends Number, we can safely call doubleValue()!
14            sum += num.doubleValue();
15        }
16        return sum / numbers.length;
17    }
18}

Valid vs Invalid Usage:

1// ✅ Valid: Integer and Double extend java.lang.Number
2StatsCalculator<Integer> intStats = new StatsCalculator<>(new Integer[]{10, 20, 30});
3StatsCalculator<Double> doubleStats = new StatsCalculator<>(new Double[]{98.5, 87.2, 91.0});
4 
5// ❌ COMPILE ERROR: String does NOT extend Number!
6// StatsCalculator<String> strStats = new StatsCalculator<>(new String[]{"A", "B"});

🔗 3. Multiple Bounds (`<T extends Class & Interface>`)

In Java, a type parameter can have multiple bounds. If one of the bounds is a class, it must be listed first, followed by interface bounds separated by &:

1// T must be a Number AND must implement Comparable<T>
2public static <T extends Number & Comparable<T>> T findMax(T a, T b) {
3    if (a.compareTo(b) >= 0) {
4        return a;
5    }
6    return b;
7}

⚠️ Common Pitfalls

1
Primitive Types as Type Arguments: You cannot use primitives like int or double directly in generics (StatsCalculator<int> ❌). You must use wrapper classes (StatsCalculator<Integer> ✅).
2
Order of Bounds: Class bounds must always precede interface bounds: <T extends Serializable & Number> ❌ compile error; <T extends Number & Serializable> ✅ correct.

Next Lesson

Wildcards in Generics: Upper Bounded (? extends) and Lower Bounded (? super)

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
Generic Classes and Generic Interfaces	Wildcards in Generics: Upper Bounded (? extends) and Lower Bounded (? super)

Practice Quiz

Test your understanding of this lesson with 1 questions. Each question has one correct answer.

Previous Next

⚡ Generic Methods and Bounded Type Parameters (<T extends Number>)

Introduction

🛠️ 1. Generic Methods Syntax

🎯 2. Bounded Type Parameters (<T extends SuperClass>)

Valid vs Invalid Usage:

🔗 3. Multiple Bounds (<T extends Class & Interface>)

⚠️ Common Pitfalls

Wildcards in Generics: Upper Bounded (? extends) and Lower Bounded (? super)

Related Lessons

Practice Quiz

Loading Page Content...

⚡ Generic Methods and Bounded Type Parameters (<T extends Number>)

Introduction

🛠️ 1. Generic Methods Syntax

🎯 2. Bounded Type Parameters (<T extends SuperClass>)

Valid vs Invalid Usage:

🔗 3. Multiple Bounds (<T extends Class & Interface>)

⚠️ Common Pitfalls

Wildcards in Generics: Upper Bounded (? extends) and Lower Bounded (? super)

Related Lessons

Practice Quiz

🎯 2. Bounded Type Parameters (`<T extends SuperClass>`)

🔗 3. Multiple Bounds (`<T extends Class & Interface>`)

🎯 2. Bounded Type Parameters (`<T extends SuperClass>`)

🔗 3. Multiple Bounds (`<T extends Class & Interface>`)