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
– Rohit Jain, Commented Jul 9, 2013 at 16:33
Sure, the equals() method returns true if both the strings have the same value -- and the arguments are both Strings. — Louis Wasserman
– Louis Wasserman, Commented Jul 9, 2013 at 16:33
A String can never be equal() to a StringBuffer (or vice versa) since they are different classes. — Code-Apprentice
– Code-Apprentice, Commented Jul 9, 2013 at 16:33

Community · Accepted Answer · 2017-05-23 12:11:17Z

8

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

edited May 23, 2017 at 12:11

CommunityBot

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answered Jul 9, 2013 at 16:32

Luiggi Mendoza

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Comments

nanofarad · Accepted Answer · 2013-07-09 16:33:15Z

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 9, 2013 at 16:33

nanofarad

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Code-Apprentice · Accepted Answer · 2013-07-09 16:37:04Z

0

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)

answered Jul 9, 2013 at 16:37

Code-Apprentice

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Code-Apprentice · Accepted Answer · 2013-07-09 16:40:04Z

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.

Mohammad Changani · Accepted Answer · 2013-07-09 16:49:59Z

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 9, 2013 at 16:49

Mohammad Changani

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