CODEWITHSUNDEEP
javaandroidstring
androidXP
androidXP

Reputation: 1719

if (String != null && !String.isEmpty()) not working for checking null

I am checking String whether it's null or not but I don't know why it's always enter into if block 

 Log.d(TAG,"String ID  "+str);
 if (str!= null && !str.isEmpty()){
    Log.d(TAG,"String ID is not Null ");                 
 } else {
    Log.d(TAG,"String ID is Null ");            
 }

I also generate logs to check String value before checking if condition and it shows like this

01-06 07:17:46.128 30321-30376/com.example D/### String Checker ###: Page ID  null
01-06 07:17:46.128 30321-30376/com.example D/### String Checker ###: Page ID is not Null

Update: Setting String to null then calling other class like this

String str = null;
new AsyncTask(searchInterfaceChat,getActivity(),String.valueOf(str)).execute(query);

Upvotes: 1

Views: 1296

Answers (5)

Pankaj Kumar
Pankaj Kumar

Reputation: 83008

While looking into your logs, it seems the value of your string is "null". your string is not null. Check into that. This observation is only for your logs

After your update of code, problem is String.valueOf(str), it is converting null to "null" which is actually a String. Only pass str, not String.valueOf(str), like new AsyncTask(searchInterfaceChat,getActivity(), str).execute(query);

Updates for little bit explanation

If you look into code String.valueOf(), it returns "null" not null if in case of null Object. So for you, it is actually "null" which is being printed correctly on the log, but making confusion. Below is code

/**
 * Returns the string representation of the {@code Object} argument.
 *
 * @param   obj   an {@code Object}.
 * @return  if the argument is {@code null}, then a string equal to
 *          {@code "null"}; otherwise, the value of
 *          {@code obj.toString()} is returned.
 * @see     java.lang.Object#toString()
 */
public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

You can read Difference between null and empty ("") Java String which not exact QA, but you can get some idea about "null" and null

Upvotes: 3

Md. Enamul Haque
Md. Enamul Haque

Reputation: 1026

Bring the null first so that It don't give error.

When str is not null it's ok

But

if str is null

and you write str!=null the it will first try to read str but it is null so it will give an error.

if (null!=str  && !str.isEmpty()){
   Log.d(TAG,"String ID is not Null ");                 
} else {
   Log.d(TAG,"String ID is Null ");            
}

Upvotes: 1

Lenoarod
Lenoarod

Reputation: 3620

if we want to know, we can look at the source code. the String isEmpty method returns true if the string length is zero. 

    public boolean isEmpty() {
        // Android-changed: Get length from count field rather than value array (see above).
        // Empty string has {@code count == 0} with or without string compression enabled.
        // return value.length == 0;
        return count == 0;
    }

the method String.valueOf will return a "null" if you object is null.

/**
     * @return  if the argument is {@code null}, then a string equal to
     *          {@code "null"}; otherwise, the value of
     *          {@code obj.toString()} is returned.
     * @see     java.lang.Object#toString()
     */
 public static String valueOf(Object obj) {
        return (obj == null) ? "null" : obj.toString();
    }

so when you use it in other class, it return the string not null.

Upvotes: 2

haresh
haresh

Reputation: 1420

Why not to use TextUtils.isEmpty() it will check both of your condition.

 if (!TextUtils.isEmpty("your string")){
Log.d(TAG,"String ID is not Null ");                 
 } else {
   Log.d(TAG,"String ID is Null ");            
    }

Also pass the string because in you case you are doing with th string class.

Tried with String s=null and s="ad" value working fine in both case :

 public class Main
 {
public static void main(String[] args) {
 String str ="ss";
 if (str!=null&&!str.isEmpty()){
System.out.println("not Null");               
} else {
      System.out.println(" Null");               
    }
}

Upvotes: 1

Scary Wombat
Scary Wombat

Reputation: 44844

You are passing the String value null as this is what will be returned by String.valueOf(str); when str is null

Upvotes: 4

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