從Java 7開始在new一個泛型(generic type)類別時型別可以用<>替代，該類別的型別決定於所分派變數宣告的型別。
例如下面的Box是一個泛型類別Box。
public class Box<T> {
private T content;
// constructor
public Box() {}
public Box(T content) {
this.content = content;
}
public T getContent() {
return content;
}
public void setContent(T content) {
this.content = content;
}
}
在Java 7以前建構式要指出泛型的型別。
public class Main {
public static void main(String[] args) {
Box<String> box1 = new Box<String>();
Box<Integer> box2 = new Box<Integer>();
box1.setContent("string");
box2.setContent(123);
Box<String> box3 = createBoxWithStringContent();
Box<Integer> box4 = createBoxWithIntegerContent();
System.out.println(box1.getContent()); // String
System.out.println(box2.getContent()); // Integer
System.out.println(box3.getContent()); // String
System.out.println(box4.getContent()); // Integer
}
public static Box<String> createBoxWithStringContent() {
return new Box<String>("hello world");
}
public static Box<Integer> createBoxWithIntegerContent() {
return new Box<Integer>(12345);
}
}
而從Java 7開始，建構式的泛型型別可以用<>(diamond operator)省略如下。
public class Main {
public static void main(String[] args) {
Box<String> box1 = new Box<>();
Box<Integer> box2 = new Box<>();
box1.setContent("string");
box2.setContent(123);
Box<String> box3 = createBoxWithStringContent();
Box<Integer> box4 = createBoxWithIntegerContent();
System.out.println(box1.getContent()); // String
System.out.println(box2.getContent()); // Integer
System.out.println(box3.getContent()); // String
System.out.println(box4.getContent()); // Integer
}
public static Box<String> createBoxWithStringContent() {
return new Box<>("hello world");
}
public static Box<Integer> createBoxWithIntegerContent() {
return new Box<>(12345);
}
}
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