public static void main(String[] args){
	String s1 = "a";
	String s2 = "b";
	String s3 = "a" + "b";//编译期优化为”ab“，常量池没有，入池
	String s4 = s1 + s2;//new String（”ab“）堆中的对象
	String s5 = "ab";//常量池中已有
	String s6 = s4.intern();//池中已有，直接返回常量池中的
	
	//问
	System.out.println(s3 == s4);//false
	System.out.println(s3 == s5);//ture
	System.out.println(s3 == s6);//ture
	
	String x2 = new String("c") + new String("d");//堆
	String x1 = "cd";//常量池
	x2.intern();//入池失败
	
	//问，如果调换了位置呢，如果是jdk1.6呢
	System.out.println(x1 == x2);//false
}