问题

How can I check whether a string is not null and not empty?

public void doStuff(String str)
{
    if (str != null && str != \"**here I want to check the \'str\' is empty or not**\")
    {
        /* handle empty string */
    }
    /* ... */
}

回答1:

What about isEmpty() ?

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

Be sure to use the parts of && in this order, because java will not proceed to evaluate the second part 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 on previous versions.

To ignore whitespace as well:

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

(since Java 11 str.trim().isEmpty() can be reduced to str.isBlank() which will also test for other Unicode white spaces)

Wrapped in a handy function:

public static boolean empty( final String s ) {
  // Null-safe, short-circuit evaluation.
  return s == null || s.trim().isEmpty();
}

Becomes:

if( !empty( str ) )

回答2:

Use org.apache.commons.lang.StringUtils

I like to use Apache commons-lang for these kinds of things, and especially the StringUtils utility class:

import org.apache.commons.lang.StringUtils;

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

if (StringUtils.isBlank(str)) {
    ...
}

回答3:

Just adding Android in here:

import android.text.TextUtils;

if (!TextUtils.isEmpty(str)) {
...
}

回答4:

To add to @BJorn and @SeanPatrickFloyd The Guava way to do this is:

Strings.nullToEmpty(str).isEmpty(); 
// or
Strings.isNullOrEmpty(str);

Commons Lang is more readable at times but I have been slowly relying more on Guava plus sometimes Commons Lang is confusing when it comes to isBlank() (as in what is whitespace or not).

Guava's version of Commons Lang isBlank would be:

Strings.nullToEmpty(str).trim().isEmpty()

I will say code that doesn't allow "" (empty) AND null is suspicious and potentially buggy in that it probably doesn't handle all cases where is not allowing null makes sense (although for SQL I can understand as SQL/HQL is weird about '').

回答5:

str != null && str.length() != 0

alternatively

str != null && !str.equals("")

str != null && !"".equals(str)

Note: The second check (first and second alternatives) assumes str is not null. It's ok only because the first check is doing that (and Java doesn't does the second check if the first is false)!

IMPORTANT: DON'T use == for string equality. == checks the pointer is equal, not the value. Two strings can be in different memory addresses (two instances) but have the same value!

回答6:

Almost every library I know defines a utility class called StringUtils, StringUtil or StringHelper, and they usually include the method you are looking for.

~~My personal favorite is Apache Commons / Lang, where in the StringUtils class, you get both the~~

StringUtils.isEmpty(String) and the
StringUtils.isBlank(String) method

(The first checks whether a string is null or empty, the second checks whether it is null, empty or whitespace only)

There are similar utility classes in Spring, Wicket and lots of other libs. If you don't use external libraries, you might want to introduce a StringUtils class in your own project.

Update: many years have passed, and these days I'd recommend using Guava's Strings.isNullOrEmpty(string) method.

回答7:

This works for me:

import com.google.common.base.Strings;

if (!Strings.isNullOrEmpty(myString)) {
       return myString;
}

Returns true if the given string is null or is the empty string.

Consider normalizing your string references with nullToEmpty. If you do, you can use String.isEmpty() instead of this method, and you won't need special null-safe forms of methods like String.toUpperCase either. Or, if you'd like to normalize "in the other direction," converting empty strings to null, you can use emptyToNull.

回答8:

How about:

if(str!= null && str.length() != 0 )

回答9:

Use Apache StringUtils' isNotBlank method like

StringUtils.isNotBlank(str)

It will return true only if the str is not null and is not empty.

回答10:

You should use org.apache.commons.lang3.StringUtils.isNotBlank() or org.apache.commons.lang3.StringUtils.isNotEmpty. The decision between these two is based on what you actually want to check for.

The isNotBlank() checks that the input parameter is:

not Null,
not the empty string ("")
not a sequence of whitespace characters (" ")

The isNotEmpty() checks only that the input parameter is

not null
not the Empty String ("")

回答11:

If you don't want to include the whole library; just include the code you want from it. You'll have to maintain it yourself; but it's a pretty straight forward function. Here it is copied from commons.apache.org

    /**
 * <p>Checks if a String is whitespace, empty ("") or null.</p>
 *
 * <pre>
 * StringUtils.isBlank(null)      = true
 * StringUtils.isBlank("")        = true
 * StringUtils.isBlank(" ")       = true
 * StringUtils.isBlank("bob")     = false
 * StringUtils.isBlank("  bob  ") = false
 * </pre>
 *
 * @param str  the String to check, may be null
 * @return <code>true</code> if the String is null, empty or whitespace
 * @since 2.0
 */
public static boolean isBlank(String str) {
    int strLen;
    if (str == null || (strLen = str.length()) == 0) {
        return true;
    }
    for (int i = 0; i < strLen; i++) {
        if ((Character.isWhitespace(str.charAt(i)) == false)) {
            return false;
        }
    }
    return true;
}

回答12:

It is a bit too late, but here is a functional style of checking:

Optional.ofNullable(str)
    .filter(s -> !(s.trim().isEmpty()))
    .ifPresent(result -> {
       // your query setup goes here
    });

回答13:

Returns true or false based on input

Predicate<String> p = (s)-> ( s != null && !s.isEmpty());
p.test(string);

回答14:

There is a new method in java-11: String#isBlank

Returns true if the string is empty or contains only white space codepoints, otherwise false.

jshell> "".isBlank()
$7 ==> true

jshell> " ".isBlank()
$8 ==> true

jshell> " ! ".isBlank()
$9 ==> false

This could be combined with Optional to check if string is null or empty

boolean isNullOrEmpty = Optional.ofNullable(str).map(String::isBlank).orElse(true);

String#isBlank

回答15:

test equals with an empty string and null in the same conditional:

if(!"".equals(str) && str != null) {
    // do stuff.
}

Does not throws NullPointerException if str is null, since Object.equals() returns false if arg is null.

the other construct str.equals("") would throw the dreaded NullPointerException. Some might consider bad form using a String literal as the object upon wich equals() is called but it does the job.

Also check this answer: https://stackoverflow.com/a/531825/1532705

回答16:

Simple solution :

private boolean stringNotEmptyOrNull(String st) {
    return st != null && !st.isEmpty();
}

回答17:

As seanizer said above, Apache StringUtils is fantastic for this, if you were to include guava you should do the following;

public List<Employee> findEmployees(String str, int dep) {
 Preconditions.checkState(StringUtils.isNotBlank(str), "Invalid input, input is blank or null");
 /** code here **/
}

May I also recommend that you refer to the columns in your result set by name rather than by index, this will make your code much easier to maintain.

回答18:

I've made my own utility function to check several strings at once, rather than having an if statement full of if(str != null && !str.isEmpty && str2 != null && !str2.isEmpty). This is the function:

public class StringUtils{

    public static boolean areSet(String... strings)
    {
        for(String s : strings)
            if(s == null || s.isEmpty)
                return false;

        return true;
    }   

}

so I can simply write:

if(!StringUtils.areSet(firstName,lastName,address)
{
    //do something
}

回答19:

You can use StringUtils.isEmpty(), It will result true if the string is either null or empty.

 String str1 = "";
 String str2 = null;

 if(StringUtils.isEmpty(str)){
     System.out.println("str1 is null or empty");
 }

 if(StringUtils.isEmpty(str2)){
     System.out.println("str2 is null or empty");
 }

will result in

str1 is null or empty

str2 is null or empty

回答20:

I would advise Guava or Apache Commons according to your actual need. Check the different behaviors in my example code:

import com.google.common.base.Strings;
import org.apache.commons.lang.StringUtils;

/**
 * Created by hu0983 on 2016.01.13..
 */
public class StringNotEmptyTesting {
  public static void main(String[] args){
        String a = "  ";
        String b = "";
        String c=null;

    System.out.println("Apache:");
    if(!StringUtils.isNotBlank(a)){
        System.out.println(" a is blank");
    }
    if(!StringUtils.isNotBlank(b)){
        System.out.println(" b is blank");
    }
    if(!StringUtils.isNotBlank(c)){
        System.out.println(" c is blank");
    }
    System.out.println("Google:");

    if(Strings.isNullOrEmpty(Strings.emptyToNull(a))){
        System.out.println(" a is NullOrEmpty");
    }
    if(Strings.isNullOrEmpty(b)){
        System.out.println(" b is NullOrEmpty");
    }
    if(Strings.isNullOrEmpty(c)){
        System.out.println(" c is NullOrEmpty");
    }
  }
}

Result:
Apache:
a is blank
b is blank
c is blank
Google:
b is NullOrEmpty
c is NullOrEmpty

回答21:

In case you are using Java 8 and want to have a more Functional Programming approach, you can define a Function that manages the control and then you can reuse it and apply() whenever is needed.

Coming to practice, you can define the Function as

Function<String, Boolean> isNotEmpty = s -> s != null && !"".equals(s)

Then, you can use it by simply calling the apply() method as:

String emptyString = "";
isNotEmpty.apply(emptyString); // this will return false

String notEmptyString = "StackOverflow";
isNotEmpty.apply(notEmptyString); // this will return true

If you prefer, you can define a Function that checks if the String is empty and then negate it with !.

In this case, the Function will look like as :

Function<String, Boolean> isEmpty = s -> s == null || "".equals(s)

Then, you can use it by simply calling the apply() method as:

String emptyString = "";
!isEmpty.apply(emptyString); // this will return false

String notEmptyString = "StackOverflow";
!isEmpty.apply(notEmptyString); // this will return true

回答22:

If you are using Spring Boot then below code will do the Job

StringUtils.hasLength(str)

回答23:

With Java 8 Optional you can do:

public Boolean isStringCorrect(String str) {
    return Optional.ofNullable(str)
            .map(String::trim)
            .map(string -> !str.isEmpty())
            .orElse(false);
}

In this expression, you will handle Strings that consist of spaces as well.

回答24:

For completeness: If you are already using the Spring framework, the StringUtils provide the method

org.springframework.util.StringUtils.hasLength(String str)

Returns: true if the String is not null and has length

as well as the method

org.springframework.util.StringUtils.hasText(String str)

Returns: true if the String is not null, its length is greater than 0, and it does not contain whitespace only

回答25:

Simply, to ignore white space as well:

if (str == null || str.trim().length() == 0) {
    // str is empty
} else {
    // str is not empty
}

回答26:

If you use Spring framework then you can use method:

org.springframework.util.StringUtils.isEmpty(@Nullable Object str);

This method accepts any Object as an argument, comparing it to null and the empty String. As a consequence, this method will never return true for a non-null non-String object.

回答27:

The better way to handle null in the string is,

str!=null && !str.equalsIgnoreCase("null") && !str.isEmpty()

In short,

str.length()>0 && !str.equalsIgnoreCase("null")

回答28:

To check on if all the string attributes in an object is empty(Instead of using !=null on all the field names following java reflection api approach

private String name1;
private String name2;
private String name3;

public boolean isEmpty()  {

    for (Field field : this.getClass().getDeclaredFields()) {
        try {
            field.setAccessible(true);
            if (field.get(this) != null) {
                return false;
            }
        } catch (Exception e) {
            System.out.println("Exception occurred in processing");
        }
    }
    return true;
}

This method would return true if all the String field values are blank,It would return false if any one values is present in the String attributes

回答29:

I've encountered a situation where I must check that "null" (as a string) must be regarded as empty. Also white space and an actual null must return true. I've finally settled on the following function...

public boolean isEmpty(String testString) {
  return ((null==testString) || "".equals((""+testString).trim()) || "null".equals((""+testString).toLowerCase()));
}

回答30:

import android.text.TextUtils;

if (!TextUtils.isEmpty(str)||!str.equalsIgnoreCase("") {
    ...
}

来源：https://stackoverflow.com/questions/3598770/check-whether-a-string-is-not-null-and-not-empty

标签

java

string

string-comparison

Check whether a string is not null and not empty

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