例子:
public static void main(String[] args) throws IOException{ String s1 = "Programming"; String s2 = new String("Programming"); String s3 = "Program"; String s4 = "ming"; String s5 = "Program" + "ming"; String s6 = s3 + s4; System.out.println(s1 == s2); //false System.out.println(s1 == s5); //true System.out.println(s1 == s6); //false System.out.println(s1 == s6.intern()); // true System.out.println(s2 == s2.intern()); // false }
为什么?
我们先看看反编译后的代码:
public static void main(String[] paramArrayOfString) throws IOException { String str1 = "Programming"; String str2 = new String("Programming"); String str3 = "Program"; String str4 = "ming"; String str5 = "Programming"; String str6 = str3 + str4; System.out.println(str1 == str2); System.out.println(str1 == str5); System.out.println(str1 == str6); System.out.println(str1 == str6.intern()); System.out.println(str2 == str2.intern()); }
在解答之前先复习一下:
1.==判断的是对象的内存地址,而equals判断的是对象内容;
2. String对象的intern方法会得到字符串对象在常量池中对应的版本的引用(如果常量池中有一个字符串与String对象的equals结果是true),如果常量池中没有对应的字符串,则该字符串将被添加到常量池中,然后返回常量池中字符串的引用;
解答:
1.s1与s2内存地址不一致,故:System.out.println(s1 == s2); //false
2.从反编译的代码看出,s5是直接赋值字符串"Programming",因为"Programming"已存在常量池中,就直接使用常量池中的字符串,故:System.out.println(s1 == s5); //true
3.实际String s6=(new StringBuilder(s3)).append(s4).toString(),是一个新的String对象,故:System.out.println(s1 == s6); //false
4.s6.intern() 意思是拿到s6字符串对象在常量池中对应的版本的引用,由于常量池中已存在"Programming"并且是在初始化s1时生成的,故:System.out.println(s1 == s6.intern()); // true
5.同理,s2.intern()返回的的字符串引用同样等于s1,故: System.out.println(s2 == s2.intern()); // false