Solution 1 :
Instead of this,
Button button = this.findViewById(R.id.SAVE);
TextView InputID = findViewById(R.id.textView);
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
please do this.
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button button = this.findViewById(R.id.SAVE);
TextView InputID = findViewById(R.id.textView);
In your code, you are setting R.id.textView and R.id.SAVE when your activity hasn’t yet set its layout via setContentView, so when you call
button.setOnClickListener{...}
it will crash because there is no button reference yet and because there is no layout yet being set, hence NullPointerException
For now, keep these 4 lines in mind and in-order.
- onCreate(Bundle savedInstanceState)
- super.onCreate(savedInstanceState)
- setContentView(R.layout.my_layout)
- findViewByid(R.id.my_view)
And to help you out further a little bit, when you encounter the dreadful NullPointerException or the so called NPE, don’t panic, just look at the left side of the . operator and try to think why its non-existent when you are calling some of its methods
Solution 2 :
Put your initialization after
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
i.e.
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button button = this.findViewById(R.id.SAVE);
TextView InputID = findViewById(R.id.textView);
Problem :
i have a button variable and is keeps crashing
here’s MainActivity.java:
package com.example.datasaving;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
Button button = this.findViewById(R.id.SAVE);
TextView InputID = findViewById(R.id.textView);
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
String msg = InputID.getText().toString();
Toast.makeText(MainActivity.this, "", Toast.LENGTH_SHORT).show();
}
});
}
}
and Here’s activity_main.xml:
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns_android="http://schemas.android.com/apk/res/android"
xmlns_app="http://schemas.android.com/apk/res-auto"
xmlns_tools="http://schemas.android.com/tools"
android_layout_width="match_parent"
android_layout_height="match_parent"
tools_context=".MainActivity">
<Button
android_id="@+id/SAVE"
android_layout_width="match_parent"
android_layout_height="wrap_content"
android_text="@string/save"
app_layout_constraintBottom_toBottomOf="parent"
app_layout_constraintEnd_toEndOf="parent"
app_layout_constraintStart_toStartOf="parent"
app_layout_constraintTop_toTopOf="parent" />
<TextView
android_id="@+id/textView"
android_layout_width="wrap_content"
android_layout_height="wrap_content"
android_hint="@string/enter_your_gmail"
android_textColorHint="#757575"
app_layout_constraintBottom_toTopOf="@+id/SAVE"
app_layout_constraintEnd_toEndOf="parent"
app_layout_constraintStart_toStartOf="parent"
app_layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>
now it gives me an error saying :
FATAL EXCEPTION: main
Process: com.example.datasaving, PID: 6961
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.datasaving/com.example.datasaving.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3921)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:4078)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:103)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2423)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loopOnce(Looper.java:233)
at android.os.Looper.loop(Looper.java:334)
at android.app.ActivityThread.main(ActivityThread.java:8348)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:582)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1065)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.example.datasaving.MainActivity.onCreate(MainActivity.java:20)
at android.app.Activity.performCreate(Activity.java:8363)
at android.app.Activity.performCreate(Activity.java:8341)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1329)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3894)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:4078)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:103)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2423)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loopOnce(Looper.java:233)
at android.os.Looper.loop(Looper.java:334)
at android.app.ActivityThread.main(ActivityThread.java:8348)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:582)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1065)
so this says that i have a null object but when i ctrl click that object it sends me back to The button what could be wrong am I doing something wrong or is it just the android studio???
