How to check a string is not null?

if(string.equals(""))
开发者_C百科{

}

How to check if the string is not null?

if(!string.equals(""))
{

}

Checking for null is done via if (string != null)

If you want to check if its null or empty - you'd need if (string != null && !string.isEmpty())

I prefer to use commons-lang StringUtils.isNotEmpty(..)

You can do it with the following code:

 if (string != null) {

 }

Checking for null is done by:

string != null

Your example is actually checking for the empty string

You can combine the two like this:

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

But null and empty are two different things

Nothing really new to add to the answers above, just wrapping it into a simple class. Commons-lang is quite all right but if all you need are these or maybe a few more helper functions, rolling your own simple class is the easiest approach, also keeping executable size down.

public class StringUtils {

  public static boolean isEmpty(String s) {
    return (s == null || s.isEmpty());
  }

  public static boolean isNotEmpty(String s) {
    return !isEmpty(s);
  }
}

Use TextUtils Method.

TextUtils.isEmpty(str) : Returns true if the string is null or 0-length. Parameters: str the string to be examined Returns: true if str is null or zero length

if(TextUtils.isEmpty(str)){
    // str is null or lenght is 0
}

Source of TextUtils class

isEmpty Method :

 public static boolean isEmpty(CharSequence str) {
        if (str == null || str.length() == 0)
            return true;
        else
            return false;
    }

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

Be sure to use the parts of && in this order, because java will not proceed to evaluating the the second if the first part of && fails, thus ensuring you will not get a null pointer exception from str.isEmpty() if str is null.

Beware, it's only available since Java SE 1.6.

You have to check str.length() == 0 or str.equals("") on previous versions.

As everyone is saying, you'd have to check (string!=null), in objects you're testing the memory pointer.

because every object is identified by a memory pointer, you have to check your object for a null pointer before testing anything else, so:

(string!=null && !string.equals("")) is good

(!string.equals("") && string !=null) can give you a nullpointerexception.

if you don't care for trailing spaces you can always use trim() before equals() so " " and "" gives you the same result

The best way to check a String is :

import org.apache.commons.lang3.StringUtils;

if(StringUtils.isNotBlank(string)){
....
}

From the doc :

isBlank(CharSequence cs) :

Checks if a CharSequence is empty (""), null or whitespace only.

You can use Predicate and its new method (since java 11) Predicate::not

You can write code to check if string is not null and not empty:

Predicate<String> notNull = Predicate.not(Objects::isNull);
Predicate<String> notEmptyString = Predicate.not(String::isEmpty);
Predicate<String> isNotEmpty = notNull.and(notEmptyString);

Then you can test it:

System.out.println(isNotEmpty.test(""));      // false
System.out.println(isNotEmpty.test(null));    // false
System.out.println(isNotEmpty.test("null"));  // true

A common way for testing null string in Java is with Optionals:

Optional.ofNullable(myString).orElse("value was null")

Optional.ofNullable(myString).ifPresent(s -> System.out.println(s));

Optional.ofNullable(myString).orElseThrow(() -> new RuntimeException("value was null"));

And to test if it is null or empty you can use Apache org.apache.commons.lang3 library that gives you the following methods:

• StringUtils.isEmpty(String) / StringUtils.isNotEmpty(String): It tests if the String is null or empty (" " is not empty)
• StringUtils.isBlank(String) / StringUtils.isNotBlank(String): Same as isEmpty bt if the String is only whitespace it is considered blank

And applied to Optional you get:

Optional.ofNullable(myString).filter(StringUtils::isNotEmpty).orElse("value was null or empty");

Try using Strings.isNullOrEmpty("") from com.google.common.base.Strings this method returns boolean value and checks for both null and empty string.

if(string != null)

or

if(string.length() == 0)

or

if(("").equals(string))

u can try this

if(string != null)