【笔记】java 泛型

java泛型有许多应用，下面挑几个重点的来说。

普通型 → 泛型的最基本应用，很简单就不说了。

通配型

class Point<K,V>
{
    private K key;
    private V value;
    
    public void setKey(K key){
        this.key = key;
    }
    
    public K getKey(){
        return this.key;
    }
    
    public void setValue(V value){
        this.value = value;
    }
    
    public V getValue(){
        return this.value;
    }
    
    public String toString(){
        return this.value.toString();
    }
}

public class Test{
    public static void main(String[] args){
        Point<Number,String> p = new Point<Number,String>();
        p.setValue("Nothing");
        fun(p);
    }
    
    public static void fun(Point<?,?> p){//重点是这里，意思是接受的对象类型不限定
        System.out.println(p);
    }
}

 

受限泛型

class Point<K>
{
    private K key;
    
    public void setKey(K key){
        this.key = key;
    }
    
    public K getKey(){
        return this.key;
    }
    
    public String toString(){
        return this.key.toString();
    }
}

public class Test{
    public static void main(String[] args){
        Point<Number> p = new Point<Number>();
        p.setKey(1.22D);
        fun1(p);
        fun2(p);
    }
    
    public static void fun1(Point<? extends Object> p){// extends 意思是接受的对象类型必须要是Object类或Object类的子类
        System.out.println(p);
    }
    
    public static void fun2(Point<? super Double> p){// super 意思是接受的对象类型必须包含有Double的子类
        System.out.println(p);
        
    }
}

同类但泛型类型不同的对象之间不能相互赋值。

class Point<K>
{
    private K key;
    
    public void setKey(K key){
        this.key = key;
    }
    
    public K getKey(){
        return this.key;
    }
    
    public String toString(){
        return this.key.toString();
    }
}

public class Test{
    public static void main(String[] args){
        Point<Number> p = new Point<Number>();
        Point<Object> p2 = null;
        p = p2;      // 这里会发生编译错误，泛型类型不等
    }
}

泛型接口

interface info<K>{//接口上定义泛型
    public K getKey();
}

class Point<K> implements info<K>//泛型接口的子类，该子类同样是泛型，子类的泛型类型与接口的泛型类型需保持一致
{
    private K key;
    
    public Point(K key){
        this.setKey(key);
    }
    
    private void setKey(K key){
        this.key = key;
    }
    
    public K getKey() {
        return this.key;
    }
    
    public String toString(){
        return this.key.toString();
    }
}

public class Test{
    public static void main(String[] args){
        info<Number> i = null;
        i = new Point<Number>(222.222);
        System.out.println(i.getKey());
    }
}

以上是对类及接口进行泛型定义，下面是对类方法进行泛型定义

class Point
{
    public <T> T getValue(T t){//注意方法中泛型的写法，与类或接口的泛型写法不大一样
        return t;
    }
}
public class Test{
    public static void main(String[] args){
        Point p = new Point();
        int t = p.getValue(111);
        System.out.println(t);
    }
}

 通过泛型方法返回泛型实例对象

 

class Info<T extends Number>{
    private T var;
    public T getVar(){
        return var;
    }
    public void setVar(T var){
        this.var = var;
    }
    
    public String toString(){
        return this.var.toString();
    }
}

public class Test{
    public static void main(String agrs[]){
        Info<Integer> i = fun(30);
        System.out.println(i.toString());
    }
    //返回的泛型对象实例必须与泛型对象保持一致
    public static <T extends Number>  Info<T> fun(T param){
        Info<T> temp= new Info<T>();
        temp.setVar(param);
        return  temp;
    }
}

 泛型数组

public class Test{
    public static void main(String agrs[]){
        
        Integer i[] = fun1(1,2,3,4,5,6);
        fun2(i);
    }
    
    public static <T> T[] fun1(T...arg){//接受参数
        return arg;//返回泛型数组
    }
    
    public static <T> void fun2(T param[]){
        System.out.println("接受泛型数组");
        for(T t:param){
            System.out.println(t);
        }
    }
}

泛型嵌套

class Info<T>{
    private T var;
    
    public Info(T var){
        this.setVar(var);
    }
    private void setVar(T var){
        this.var = var;
    }
    
    public T getVar(){
        return this.var;
    }
}

class Info2<S>{
    private S info;
    public Info2(S info){
        this.setInfo(info);
    }
    private void setInfo(S info){
        this.info = info;
    }
    public S getInfo(){
        return this.info;
    }
}

public class Test{
    public static void main(String agrs[]){
        Info2<Info<String>> d = null;
        Info<String> i = null;
        i = new Info<String>("Test");
        d = new Info2<Info<String>>(i);
        System.out.println("content1:"+i.getVar().toString());
        System.out.println("content2:"+d.getInfo().getVar().toString());
    }
}