Java中字符串相等的测试
Java中测试字符串是否相等的方法是.equals()，如果使用C++中惯用的==运算符，则有可能出错。对于字符串常量，JVM会合理安排它们以便能够共享，这时可以使用==来判定相等性；而字符串如果是+或substring等操作的结果，则千万别用==。看下面程序
/*
* Copyright (c) Ian F. Darwin, http://www.darwinsys.com/, 1996-2002.
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* $Id: LICENSE,v 1.8 2004/02/09 03:33:38 ian Exp $
* 2005/04/03 Modified By DotJox
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public class Equality {
public static void main(String[] args) {
new Equality().run();
}
public void run() {
String x = "hello";
String y = "hello";
// Assuming Equality uses a String constant, this prints true,true
compare(x, y);
// A "new String" is uniquely created, so this prints false, true
compare(x, new String(y));
// substring操作生成了一个新字符串, prints false, true
compare(x, "hello world".substring(0, 5));
// The intern() operator returns a string from the pool of unique strings, so this should print true, true.
compare(x, new String(x).intern());
/*很奇怪，"he"+"llo"合并字符串操作没生成新的字符串？（The string concatenation operator produces a new String object that contains the concatenation of its operands; the characters of the left operand precede the characters of the right operand in the newly created string.）但输出却是true,true*/
compare(x,"he"+"llo");
}
public void compare(String s1, String s2) {
System.out.print("==: ");
System.out.println(s1 == s2);
System.out.println(".equals():" + s1.equals(s2));
System.out.println();
}
}
PS：当一个String实例str调用intern() 方法时，Java查找字符串池中是否有相同Unicode的字符串常量，如果有，则返回其的引用， 如果没有，则在字符串池中增加一个Unicode等于str的字符串并返回它的引用。字符串池最初是空的，由String类在私下维护。
程序输出：
所以，千万不要在Java中使用==来测试字符串的相等性。它只是用来确定字符串是否存储在相同的内存位置，而对于两个相同的字符串存储在不同的位置是完全有可能的。