Chapter 74: Generic Method in Java

Generic classes (Chapter 73) tie T to the whole object. Generic methods attach type parameters to one method—handy for static helpers that must work on many element types without duplicating code.

Bounded type parameters deepen in Chapter 75.

1. Topic title

Generic method: <T> before the return type, inference fills it in at the call site

2. What it means

static <T> void swapEnds(List<T> list) {
    if (list.size() < 2) {
        return;
    }
    T tmp = list.get(0);
    list.set(0, list.get(list.size() - 1));
    list.set(list.size() - 1, tmp);
}

<T> introduces T only for swapEnds. Callers can write swapEnds(strings) or swapEnds(nums)—the compiler infers T from the argument list.

3. Why it is used

Library-style utilities—Collections.sort, Arrays.asList, List.copyOf all lean on generic methods so call sites stay clean.

4. Mental hook

If a generic class is a whole kitchen themed around vegan food, a generic method is a single blender attachment that fits many jars—borrow once, reuse often.

5. Java code example

import java.util.ArrayList;
import java.util.List;
 
public class GenericMethodDemo {
    static <T> T middle(List<T> items) {
        return items.get(items.size() / 2);
    }
 
    public static void main(String[] args) {
        List<String> letters = new ArrayList<>(List.of("a", "b", "c", "d", "e"));
        System.out.println(middle(letters));
 
        List<Integer> nums = new ArrayList<>(List.of(10, 20, 30));
        System.out.println(middle(nums));
    }
}

Two different T inferences—String then Integer—from the same method body.

6. Explanation of code

If inference fails (rare in simple cases), call with explicit type witness: GenericMethodDemo.<String>middle(letters)—ugly but clarifying for the compiler in edge cases.

7. Common mistakes

Thinking <T> on a method shadows the class <T> accidentally—legal but confusing; rename one (U) when both appear.

Using raw List in a generic method signature—defeats safety; keep List<T>.

8. Best practices

Prefer List.copyOf / List.of when building immutable snapshots to pass into generic utilities.

Keep generic methods short—if the method grows huge, maybe a small generic class carries state clearer.

9. Small practice task

Write <T> List<T> firstTwo(List<T> source) returning a new ArrayList containing at most the first two elements. Test with List.of(1,2,3) and List.of("x").

Beginner tip

Read static <T> T middle(...) aloud as “for any type T, middle returns a T.” The <T> is the introducer; T later is the use.