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

Question

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);

@ScaryWombat is just to show that it's string to avoid confusion — androidXP, Jan 06 '20 at 02:11
write your code as `new AsyncTask(searchInterfaceChat,getActivity(), str).execute(query);` — Pankaj Kumar, Jan 06 '20 at 02:23
`System.out.println(String.valueOf(str));` will print the String *null* — Scary Wombat, Jan 06 '20 at 02:24

score 4 · Answer 1 · answered Jan 06 '20 at 02:26

4

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

answered Jan 06 '20 at 02:26

Scary Wombat

44,617
6
35
64

Can you tell me the reason behind this even when in log it's says string is null – androidXP Jan 06 '20 at 02:29
1

The log is simply printing out the String value `null` - the same as if you had declared `String str = "null";` – Scary Wombat Jan 06 '20 at 02:30
it's not `String str = "null" its `String = null` without any quotes – androidXP Jan 06 '20 at 02:32

Pankaj Kumar · Accepted Answer · 2020-01-07T02:26:56.007

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

The output before the `if` is `Page ID null`, so I think that your second assumption is not correct. — Scary Wombat, Jan 06 '20 at 02:18
*it seems value of your string is "null"* - yep as per my comment below the main question. — Scary Wombat, Jan 06 '20 at 02:21
Can you tell me the reason behind this even when in log it says string is null — androidXP, Jan 06 '20 at 02:26
No log is saying "null" not null. Understand the difference between both. null denotes a string without memory, but "null" denotes string is not empty and it has value "null". — Pankaj Kumar, Jan 06 '20 at 02:29

Lenoarod · Answer 3 · 2020-01-06T02:32:02.707

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.

haresh · Answer 4 · 2020-01-06T02:27:12.757

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");               
    }
}

I know this way but i need to know why i am getting into if block — androidXP, Jan 06 '20 at 02:13

Md. Enamul Haque · Answer 5 · 2020-01-06T03:00:13.783

1

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 ");            
}

edited Jan 06 '20 at 03:00

answered Jan 06 '20 at 02:20

Md. Enamul Haque

926
8
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if (String != null && !String.isEmpty()) not working for checking null

5 Answers5