利用有限通配符提供API靈活性

import com.sun.org.apache.xerces.internal.impl.dv.dtd.NMTOKENDatatypeValidator;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;

/**
 * Created by kaizige on 2017/7/3.
 */
public class Stack2<E> {
    private Object[] eles;
    private int size=0;
    private static  final int CAPATY=10;
    @SuppressWarnings("unchecdked")
    public Stack2(){
        eles=new Object[CAPATY];
    }
    public void push(E o){
        eles[size++]=o;
    }
    public E pop(){
        if(size==0){
            throw new RuntimeException("沒有元素");
        }
        E o=(E) eles[--size];
        eles[size]=null;
        return o;
    }
    public void pushAll(Iterator<? extends E> src){
            while(src.hasNext()){
                push(src.next());
        }
    }
    public void popAll(Collection<? super E > des){
        des.add(pop());
    }
}

顯示的類型參數：

public class FunctionFactory {
    public static <E> Set<E> union(Set<? extends  E> s1, Set<? extends  E> s2){
        Set<E> r=new HashSet<E>(s1);
        r.addAll(s2);
        return r;
    }
    public static void main(String[] args) {
       Set<Integer> s1=new HashSet<Integer>();
        s1.add(2);
        Set<Double> s2=new HashSet<Double>();
        s2.add(2.3);
        Set<Number> s=union(s1,s2);
    }