JAVA语法题

import java.util.*;

public class Birthdays {
    public static void main(String[] args){
    	Map<Friends, String> hm = new HashMap<Friends,String>();
    	Friends f1 = new Friends("Charis");
    	Friends f2 = new Friends("Darling");
    	hm.put(f1, "summer 2009");
    	hm.put(f2, "spring 2002");
    	System.out.println(hm.get(f1));
    	Friends f3 = new Friends("Darling");
    	System.out.println(hm.get(f3));
    }
}
class Friends{
	String name;
	Friends(String n){
		name = n;
	}
}

结果:

summer 2009
null

2. 　enum用法　

public class WeatherTest {
    static Weather w;
    public static void main(String[] args){
    	System.out.println(w.RAINY.count+" "+w.Sunny.count+" ");
    }
}
enum Weather{
	RAINY, Sunny;
	int count = 0;
	Weather(){
		System.out.print("c ");
		count++;
	}
}

结果

c c 1 1

public class WeatherTest {
    static Weather w;
    public static void main(String[] args){
    	System.out.println(w.RAINY+" "+w.Sunny+" ");
    }
}
enum Weather{
	RAINY(1), Sunny(2);
	private  int n;
	private Weather(int n){
		this.n=n;
	}
	@Override
        public String toString() {
             return String.valueOf(this.n);
        }
}

结果

1 2

枚举的遍历:

public class WeatherTest {
	enum Weather{
		RAINY(1), Sunny(2);
		private  int n;
		private Weather(int n){
			this.n=n;
		}
		 @Override
	    public String toString() {
	        return String.valueOf(this.n);
	    }
	}
    public static void main(String[] args){
    	Weather[] allW = Weather.values();
    	for(Weather w: allW)
    	System.out.print(w+" ");
    }
}

结果同上, 1 2

3. 数值类型题

public class Dec26 {

    public static void main(String[] args){
    	short a1=6;
    	new Dec26().go(a1);
    	new Dec26().go(new Integer(7));
    }

    void go(short x){
    	System.out.print("S ");
    }
    void go(Long x){
    	System.out.print("L ");
    }
    void go(int x){
    	System.out.print("i ");
    }
    void go(Number n){
    	System.out.print("N ");
    }

}

结果:

S N

4. switch, case　　

public class Humping {
    public static void main(String[] args) {
    	String r="-";
    	char[] c={‘a‘,‘b‘,‘c‘,‘z‘};
    	for(char c1:c)
    		switch(c1){
    			case ‘a‘:
    				r+="a";
    			case ‘b‘:
    				r+="b";
    				break;
    			default:
    				r+="X";
    			case ‘z‘:
    				r+="z";
    		}
    		System.out.println(r);
    }
}

结果:

-abbXzz

5. HashMap键值为空的问题

HashMap：适用于在Map中插入、删除和定位元素。 
Treemap：适用于按自然顺序或自定义顺序遍历键(key)。

两者都是非线程安全，前者无排序，后者会自动排序

Ø  HashMap里面存入的键值对在取出的时候是随机的，它根据键的HashCode值存储数据,根据键可以直接获取它的值，具有很快的访问速度。在Map 中插入、删除和定位元素，HashMap是最好的选择。

Ø  TreeMap取出来的是排序后的键值对。插入、删除需要维护平衡会牺牲一些效率。但如果要按自然顺序或自定义顺序遍历键，那么TreeMap会更好。

本测试增加和查找功能，HashMap比TreeMap的效率要高。

import java.util.*;
public class Garage {

    public static void main(String[] args) {
    	Map<String,String> hm=new HashMap<String,String>();
    	String[] k={null,"2","3",null,"5"};
    	String[] v={"a","b","c","d","e"};
    	for(int i=0;i<5;i++){
    		hm.put(k[i],v[i]);
    		System.out.print(hm.get(k[i])+" ");
    	}
    	System.out.print(hm.size()+" "+hm.values()+"\n");
    }
}

结果

a b c d e 4 [c, d, b, e]

换成TreeMap报错, 所以 TreeMap键值不允许空　

null和" "的区别:

import java.util.*;
public class Garage {

    public static void main(String[] args) {
    	Map<String,String> hm=new HashMap<String,String>();
    	String[] k={null,"2","3"," ","5"};
    	String[] v={"a","b","c","d","e"};
    	for(int i=0;i<5;i++){
    		hm.put(k[i],v[i]);
    		System.out.print(hm.get(k[i])+" ");
    	}
    	System.out.print(hm.size()+" "+hm.keySet()+" "+hm.values()+"\n");
    }
}

结果

a b c d e 5 [3, null, 2,  , 5] [c, a, b, d, e]