Java · Theory

Java Generics

Theory

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Java Generics

Generics keep types safe: List<String> can only hold String. Without them, a raw List can mix Integer and String and crash later when you cast.

Write List<String> names = new ArrayList<>();. The compiler checks add. At runtime, generics are mostly erased (type erasure) — don’t expect new T() easily.

Use them on collections always. Don’t ignore the compiler warning for raw types.

Let's take this on the board with one tiny Main class — no extra files. Java Generics — output: Asha. Box<String> holds only String — generics catch wrong types at compile time. Start from main, go line by line, and stop at each print. That output is the proof for Generics.

Exam tip

Show List<String> vs raw List. Say compiler catches the wrong add.

Example

class Box<T> {
  T value;
  Box(T value) { this.value = value; }
}
public class Main {
  public static void main(String[] args) {
    Box<String> b = new Box<>("Asha");
    System.out.println(b.value);
  }
}

Java Generics — output: Asha. Box<String> holds only String — generics catch wrong types at compile time.

Short notes

DefGenerics = type-safe collections. List<String> only holds String.
RuleAlways write the type parameter. Avoid raw List.
RememberType erasure at runtime. Compiler checks at compile time.
TrapRaw types + later ClassCastException.

Java · Theory

Java Generics

← All stacks

Theory

226/270

Java Generics

Generics keep types safe: List<String> can only hold String. Without them, a raw List can mix Integer and String and crash later when you cast.

Write List<String> names = new ArrayList<>();. The compiler checks add. At runtime, generics are mostly erased (type erasure) — don’t expect new T() easily.

Use them on collections always. Don’t ignore the compiler warning for raw types.

Exam tip

Show List<String> vs raw List. Say compiler catches the wrong add.

Example

class Box<T> {
  T value;
  Box(T value) { this.value = value; }
}
public class Main {
  public static void main(String[] args) {
    Box<String> b = new Box<>("Asha");
    System.out.println(b.value);
  }
}

Java Generics — output: Asha. Box<String> holds only String — generics catch wrong types at compile time.

Short notes

DefGenerics = type-safe collections. List<String> only holds String.
RuleAlways write the type parameter. Avoid raw List.
RememberType erasure at runtime. Compiler checks at compile time.
TrapRaw types + later ClassCastException.

Java Generics

Example

Short notes

Questions

Java Generics

Example

Short notes

Questions