Java String Buffer

Question

How does a StringBuffer handle strings internally? I tried running the below example and got the answer as "String are unequal".

From what I know is that the equals() returns true if both the strings have the same value. So what is happening in this case?

class StringBufferTest {

public static void main(String[] args) {
    String newString = "HelloWorld";
    StringBuffer buffer = new StringBuffer(newString);

    if (buffer.equals(newString)) {
        System.out.println("Strings are equal");
    } else {
        System.out.println("String are unequal");
    }

  }
}

You are comparing a `StringBuffer` instance to a `String` instance. — Rohit Jain, Jul 09 '13 at 16:33
Sure, the `equals()` method returns `true` if both the strings have the same value -- and the arguments are _both `String`s._ — Louis Wasserman, Jul 09 '13 at 16:33
A `String` can never be `equal()` to a `StringBuffer` (or vice versa) since they are different classes. — Code-Apprentice, Jul 09 '13 at 16:33

score 8 · Accepted Answer · edited May 23 '17 at 12:11

You're comparing an instance of StringBuffer with an instance of String, which won't give the desired results for being different types. Note that StringBuffer will use the plain Object#equals since it doesn't override it (noted by StringBuffer JavaDoc against String#equals that indeed overrides it). You must compare the String content of the buffer, not the object reference of your buffer variable:

if (buffer.toString().equals(newString)) {
    //...
}

Also, from Java 5, it would be better using StringBuilder instead of StringBuffer. More info about this: Difference between StringBuilder and StringBuffer

score 2 · Answer 2 · answered Jul 09 '13 at 16:33

2

equals( returns false when two objects being compared are not the same type. No matter the contained string, StringBuffer is a different type than String and the comparison will never be true.

buffer.toString.equals(newString) should work.

answered Jul 09 '13 at 16:33

nanofarad

40,330
4
86
117

score 0 · Answer 3 · edited Jul 09 '13 at 16:40

You are comparing:

if (buffer.equals(newString))

Which is actually comparing a StringBuffer object type to a String object type. That will obviously return false.

In order to get true returned from equals method you need to compare objects of same types.

As per official doc:

public boolean equals(Object obj)
Indicates whether some other object is "equal to" this one.
The equals method implements an equivalence relation on non-null object references:

It is reflexive: for any non-null reference value x, x.equals(x) should return true.
It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
For any non-null reference value x, x.equals(null) should return false.

score 0 · Answer 4 · answered Jul 09 '13 at 16:37

The equals() contract implies that x.equals(y) returns true if and only if x.getClass() == y.getClass(). In other words, x and y must be instances of the same class. In this case, you are trying to compare a StringBuffer to a String. Comparing instances of two different classes will always be false for a well-behaved equals() method.

In order to do a meaningful comparison, you need to get the contents of the StringBuffer:

buffer.toString().equals(newString)

score 0 · Answer 5 · answered Jul 09 '13 at 16:49

0

can not equals a String Object with a StringBuffer Object, use:

buffer.toString().equals(newString)

and better use StringBuilder, because StringBuffer not TheredSafe

answered Jul 09 '13 at 16:49

Mohammad Changani

474
3
11

Java String Buffer

5 Answers5