I can not understand why code below returns "false"
String x="Hello World";
String z=" Hello World".trim();
System.out.println(x==z); //false
I have read that "Strings are immutable and literals are pooled".After executing trim(), z will be z="Hello World" and then why output is not true?
It's because Strings are immutable! Because of that the trim() method returns a new instance of String which has a different reference. You can see it by watching the source code.
public String trim() {
int len = value.length;
int st = 0;
char[] val = value;
while ((st < len) && (val[st] <= ' ')) {
st++;
}
while ((st < len) && (val[len - 1] <= ' ')) {
len--;
}
return ((st > 0) || (len < value.length)) ? substring(st, len) : this;
}
public String substring(int beginIndex, int endIndex) {
if (beginIndex < 0) {
throw new StringIndexOutOfBoundsException(beginIndex);
}
if (endIndex > value.length) {
throw new StringIndexOutOfBoundsException(endIndex);
}
int subLen = endIndex - beginIndex;
if (subLen < 0) {
throw new StringIndexOutOfBoundsException(subLen);
}
return ((beginIndex == 0) && (endIndex == value.length)) ? this
: new String(value, beginIndex, subLen); // new instance!
}
You're comparing the pointers to the objects, which will be different. For strings you should use:
x.equals(z)
x and y are different String objects, its only the contents that are the same. To compare the contents for equality you have to use equals().