Difference between substring and string in java

Question

class test {

public static void main(String[] args) {
    String a = "Hiabc";
    String b = "abc";
    String c = "abc";
    System.out.println(a.substring(2,5)==b);
    System.out.println(b==c);
}
}

output:
false
true

As far as I understand, Java's == look for addresses of the two compared objects. Yet, I don't understand why b==c is true because they must have different addresses. Also, if b==c, then why is a.substring(2,5), which is "abc" == b false?

Because `substring` creates a **new object**. `b` and `c` refer to the *same* String in the pool. — Maroun, Oct 05 '14 at 08:42
could you explain more about the "pool"? You mean Java has all library of strings? — foothill, Oct 05 '14 at 08:55
Google it, there are infinite resources about Java Strings Pool. — Maroun, Oct 05 '14 at 08:57

score 1 · Answer 1 · edited Apr 12 '17 at 07:31

As far as I understand, Java's == look for addresses of the two compared objects.

Almost correct. Java uses references not addresses, e.g. a reference is strongly typed: See https://softwareengineering.stackexchange.com/questions/141834/how-is-a-java-reference-different-from-a-c-pointer

why b==c is true

The variable b and c are initialized with the same string reference, because they reference the same string literal "abc".

why doesn't substring expression reference the string literal "abc"?

Because the javadoc of substring() says

Returns a new string that is a substring of this string.

So the method does not ensure that a reference to a string from the constant pool is returned. If you want this you have to do

a.substring(2,5).intern()

and than

a.substring(2,5).intern()==b

will be true.

why doesn't substring expression reference the string literal "abc"? — foothill, Oct 05 '14 at 08:54

score 0 · Answer 2 · answered Oct 05 '14 at 08:41

Never compare strings with == in Java. Use .equal or .equalIgnoreCase methods instead.

The difference between a.substring(2,5) and b is because substring created another String object and it's on a different location than b. It's not about the characters ...

Difference between substring and string in java

2 Answers2