Strange Behavior of Empty String in JAVA

Question

I am working on a Existing code and i found a Strange Behavior while checking a String is null or Empty. The existing code :

System.out.println("Length: " + value.length());
if (value != null && value != "") {
   //code
}

Output:

Length: 0

But the if statement becomes true and its executing the code.

When i replaced the if statement with this one:

if (value != null && value.length() > 0) {
   //code
}

It works perfectly. Any idea why the previous one failed and Why value != "" returns true?

score 3 · Accepted Answer · answered Feb 16 '16 at 11:05

3

Try to use equals method like this:

   if (value != null && !"".equals(value)) {

answered Feb 16 '16 at 11:05

Abdelhak

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Ok but why value != "" returns true ? – st_ahmed Feb 16 '16 at 11:08
1

@st_ahmed because == and != comparing references but not content of string. – Rustam Feb 16 '16 at 11:09
@st_ahmed Ofcourse you're right Rustam, and if you want to compare values use equals() – Abdelhak Feb 16 '16 at 11:14
@Rustam thanks for explaining. – st_ahmed Feb 16 '16 at 11:17
1

`value != null` is unnecessary given your use of the Yoda expression. – Bathsheba Feb 16 '16 at 12:12

score 2 · Answer 2 · 2016-02-16T11:09:27.507

2

Strings are not compared like this:

value != ""

Use equals:

!"".equals(value)

or isEmpty:

!value.isEmpty()

Because String is immutable

edited Feb 16 '16 at 11:09

answered Feb 16 '16 at 11:05

KAD · Answer 3 · 2016-02-16T11:15:03.390

1

When using == you are comparing the references which are not equal, that's why the expression evaluates to true.

You can use StringUtils.isNotEmpty

if (StringUtils.isNotEmpty(value)) {
...
}

true - if the String is not empty and not null

Reference

edited Feb 16 '16 at 11:15

answered Feb 16 '16 at 11:08

KAD

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Plus one, that's pretty, and clearer than my Yoda trick. – Bathsheba Feb 16 '16 at 11:15

score 1 · Answer 4 · answered Feb 16 '16 at 11:08

1

You should compare with value.isEmpty(). According to the source code, it will also compare the length.

What you are doing with != is comparing references, not equality.

answered Feb 16 '16 at 11:08

Marcelo

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score 0 · Answer 5 · answered Feb 16 '16 at 11:06

0

String is an Object in java, so you can't use == and != with it, because == and != comparing references, which is not equal. Use "".equals(value)

answered Feb 16 '16 at 11:06

TEXHIK

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score 0 · Answer 6 · edited May 23 '17 at 11:50

0

value != ""

check for reference equality not the value equality.

if value is created like this:

String  value=new String("");

then there is a high chance that the condition will fail.

For example if you have another String value2

String value2=new String("");

then

  value==value2  // is false.

Have a look at this How do I compare strings in Java?

edited May 23 '17 at 11:50

Community

1
1

answered Feb 16 '16 at 11:07

Saif

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score 0 · Answer 7 · answered Feb 16 '16 at 11:11

This is because you might have created value variable using new operator:

String value = new String();

So now if you use following :

System.out.println("Length: " + value.length());
if (value != null && value != "") {
     System.out.println("hi")
}

It will print hi as value is from heap and literal "" is from string pool and you are making reference comparison. Use equals method instead .

score 0 · Answer 8 · answered Feb 16 '16 at 11:13

Your best bet is to use value != null && !value.isEmpty(), although you can ace this with the Yoda Expression

"".equals(value)

which saves you typing out the explicit check for null.

You can't use == since that would compare the object reference values (which might be different even if the string looks the same), not the string contents.

Strange Behavior of Empty String in JAVA

8 Answers8