Can we rely on String.isEmpty for checking null condition on a String in Java?

Question

No, absolutely not - because if acct is null, it won't even get to isEmpty... it will immediately throw a NullPointerException.

Your test should be:

if (acct != null && !acct.isEmpty())

Note the use of && here, rather than your || in the previous code; also note how in your previous code, your conditions were wrong anyway - even with && you would only have entered the if body if acct was an empty string.

Alternatively, using Guava:

if (!Strings.isNullOrEmpty(acct))

Use StringUtils.isEmpty instead, it will also check for null.

Examples are:

 StringUtils.isEmpty(null)      = true
 StringUtils.isEmpty("")        = true
 StringUtils.isEmpty(" ")       = false
 StringUtils.isEmpty("bob")     = false
 StringUtils.isEmpty("  bob  ") = false

See more on official Documentation on String Utils.

You can't use String.isEmpty() if it is null. Best is to have your own method to check null or empty.

public static boolean isBlankOrNull(String str) {
    return (str == null || "".equals(str.trim()));
}

No, the String.isEmpty() method looks as following:

public boolean isEmpty() {
    return this.value.length == 0;
}

as you can see it checks the length of the string so you definitely have to check if the string is null before.

I think the even shorter answer that you'll like is: StringUtils.isBlank(acct);

From the documentation: http://commons.apache.org/proper/commons-lang/javadocs/api-2.6/org/apache/commons/lang/StringUtils.html#isBlank%28java.lang.String%29

isBlank
public static boolean isBlank(String str)
Checks if a String is whitespace, empty ("") or null.

 StringUtils.isBlank(null)      = true
 StringUtils.isBlank("")        = true
 StringUtils.isBlank(" ")       = true
 StringUtils.isBlank("bob")     = false
 StringUtils.isBlank("  bob  ") = false
 
Parameters:
str - the String to check, may be null
Returns:
true if the String is null, empty or whitespace

String s1=""; // empty string assigned to s1 , s1 has length 0, it holds a value of no length string

String s2=null; // absolutely nothing, it holds no value, you are not assigning any value to s2

so null is not the same as empty.

hope that helps!!!