Type
There is nothing particulary complex about the concept of generic types.
/**
* A generic type is a special kind of variable ...
* with whatever type you pass in.
*/
package com.minte9.collections.generics;
public class Type {
public static void main(String[] args) {
A a = new A();
a.set(10);
System.out.println(a.get()); // 10
a.set("John");
System.out.println(a.get()); // John
Box<Integer> b = new Box<>();
b.set(10);
System.out.println(b.get()); // 10
// b.set("John"); // Error: not applicable ...
}
}
class A { // non generic class
private Object obj;
public void set(Object o) {
obj = o;
}
public Object get() {
return obj;
}
}
class Box<T> { // generic class (type)
private T t;
public void set(T t) {
this.t = t;
}
public T get() {
return t;
}
}
Multiple
You can use multiple generic type parameters.
/**
* A generic type may have multiple type parameters.
* Each parameter must be unique.
*/
package com.minte9.collections.generics;
public class Multiple {
public static void main(String[] args) {
Box<Integer, Integer> box = new Box<>();
box.set(10);
System.out.println(box.get()); // 10
}
static class Box<T, U> {
private T t;
public void set(T t) {
this.t = t;
}
public T get() {
return t;
}
}
// static class Word<T, T> {} // Error: Duplicate type parameter T
}
Limitations
Generics types cannot be static (or used with primitives).
/**
* Generic type params can't be static.
*
* Because static field is shared by all objects ...
* what's the actual type of t?
*
* Generics cannot be used with primitives.
*/
package com.minte9.collections.generics;
public class Limitations {
public static void main(String[] args) {
Box<Integer> b1 = new Box<>();
Box<String> b2 = new Box<>();
System.out.println(b1.hashCode());
System.out.println(b2.hashCode());
// Box<int> box = new Box<int>(); // not allowed!
}
static class Box<T> {
//static T t; // Error: static reference to non-static T
}
}
Last update: 619 days ago
