当我将按钮放在活动上时，Android 应用程序崩溃，空活动工作

Question

**When i click button app crash At start, when i did not put anything on activity i mean its only like "hello world" it worked, but when i put 3 buttons on activity they did`t work anymore. **

package com.example.administracijazaposlenih;

import androidx.appcompat.app.AppCompatActivity;

import android.app.AlertDialog;
import android.content.Intent;
import android.media.Image;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

import org.w3c.dom.Text;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        final EditText editText = (EditText) findViewById(R.id.editText);
        TextView korisnicko = (TextView) findViewById(R.id.korisnicko);
        final EditText editText4 = (EditText) findViewById(R.id.editText4);
        TextView korisnicko2 = (TextView) findViewById(R.id.korisnicko2);
        ImageView slika = (ImageView) findViewById(R.id.imageView);
        Button button = (Button) findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                /* ime = editText.toString();
                String sifra = editText4.toString();*/
                if(editText4.getText().toString().equals("admin") && editText.getText().toString().equals("nikola")){
                Intent startIntent = new Intent(getApplicationContext(), Glavna.class);
                startActivity(startIntent);
                }
                else
                {
                    Toast.makeText(getApplicationContext(), "Pogresni podaci",Toast.LENGTH_SHORT).show();
                }



            }
        });
    }
}

This is main activity, and button open this activity"

package com.example.administracijazaposlenih;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.content.Intent;

public class Glavna extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_glavna);

        Button btn1 = (Button) findViewById(R.id.button);
        Button btn2 = (Button) findViewById(R.id.button2);
        Button btn3 = (Button) findViewById(R.id.button5);

        btn1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent prikaziUnos = new Intent(getApplicationContext(),Unos.class);
                startActivity(prikaziUnos);
            }
        });

        btn2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent prikaziBrisanje = new Intent(getApplicationContext(),Brisanje.class);
                startActivity(prikaziBrisanje);
            }
        });

        btn3.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent prikaziPregled = new Intent (getApplicationContext(), Pregled.class);
                startActivity(prikaziPregled);
            }
        });

    }
}

Here is logcat

2020-01-22 17:03:59.578 513-513/? E/Zygote: isWhitelistProcess - Process is Whitelisted
2020-01-22 17:03:59.579 513-513/? E/Zygote: accessInfo : 1
2020-01-22 17:03:59.584 513-513/? I/acijazaposleni: Late-enabling -Xcheck:jni
2020-01-22 17:03:59.858 513-563/? D/vndksupport: Loading /vendor/lib64/egl/libGLES_mali.so from current namespace instead of sphal namespace.
2020-01-22 17:03:59.908 513-563/? D/libEGL: loaded /vendor/lib64/egl/libGLES_mali.so
2020-01-22 17:03:59.956 513-513/? I/DecorView: createDecorCaptionView >> DecorView@ffa7d5a[], isFloating: false, isApplication: true, hasWindowDecorCaption: false, hasWindowControllerCallback: true
2020-01-22 17:04:00.054 513-513/? W/acijazaposleni: Accessing hidden method Landroid/view/View;->computeFitSystemWindows(Landroid/graphics/Rect;Landroid/graphics/Rect;)Z (light greylist, reflection)
2020-01-22 17:04:00.055 513-513/? W/acijazaposleni: Accessing hidden method Landroid/view/ViewGroup;->makeOptionalFitsSystemWindows()V (light greylist, reflection)
2020-01-22 17:04:00.253 513-513/? D/OpenGLRenderer: Skia GL Pipeline
2020-01-22 17:04:00.257 513-513/? D/EmergencyMode: [EmergencyManager] android createPackageContext successful
2020-01-22 17:04:00.284 513-513/? D/InputTransport: Input channel constructed: fd=61
2020-01-22 17:04:00.284 513-513/? D/ViewRootImpl@ec43217[MainActivity]: setView = DecorView@ffa7d5a[MainActivity] TM=true MM=false
2020-01-22 17:04:00.301 513-513/? D/ViewRootImpl@ec43217[MainActivity]: dispatchAttachedToWindow
2020-01-22 17:04:00.338 513-513/? D/ViewRootImpl@ec43217[MainActivity]: Relayout returned: old=[0,0][1440,2960] new=[0,0][1440,2960] result=0x7 surface={valid=true 520902209536} changed=true
2020-01-22 17:04:00.370 513-589/? I/ConfigStore: android::hardware::configstore::V1_0::ISurfaceFlingerConfigs::hasWideColorDisplay retrieved: 0
2020-01-22 17:04:00.371 513-589/? I/ConfigStore: android::hardware::configstore::V1_0::ISurfaceFlingerConfigs::hasHDRDisplay retrieved: 0
2020-01-22 17:04:00.371 513-589/? I/OpenGLRenderer: Initialized EGL, version 1.4
2020-01-22 17:04:00.371 513-589/? D/OpenGLRenderer: Swap behavior 2
2020-01-22 17:04:00.384 513-589/? D/mali_winsys: EGLint new_window_surface(egl_winsys_display *, void *, EGLSurface, EGLConfig, egl_winsys_surface **, EGLBoolean) returns 0x3000
2020-01-22 17:04:00.384 513-589/? D/OpenGLRenderer: eglCreateWindowSurface = 0x793f857900, 0x794830f010
2020-01-22 17:04:00.461 513-589/? D/vndksupport: Loading /vendor/lib64/hw/android.hardware.graphics.mapper@2.0-impl.so from current namespace instead of sphal namespace.
2020-01-22 17:04:00.649 513-513/? D/ViewRootImpl@ec43217[MainActivity]: MSG_RESIZED: frame=Rect(0, 0 - 1440, 2960) ci=Rect(0, 96 - 0, 0) vi=Rect(0, 96 - 0, 0) or=1
2020-01-22 17:04:00.650 513-513/? D/ViewRootImpl@ec43217[MainActivity]: MSG_WINDOW_FOCUS_CHANGED 1 1
2020-01-22 17:04:00.656 513-513/? D/InputMethodManager: prepareNavigationBarInfo() DecorView@ffa7d5a[MainActivity]
2020-01-22 17:04:00.656 513-513/? D/InputMethodManager: getNavigationBarColor() -855310
2020-01-22 17:04:00.673 513-513/? D/InputMethodManager: prepareNavigationBarInfo() DecorView@ffa7d5a[MainActivity]
2020-01-22 17:04:00.673 513-513/? D/InputMethodManager: getNavigationBarColor() -855310
2020-01-22 17:04:00.673 513-513/? V/InputMethodManager: Starting input: tba=com.example.administracijazaposlenih ic=null mNaviBarColor -855310 mIsGetNaviBarColorSuccess true , NavVisible : true , NavTrans : false
2020-01-22 17:04:00.673 513-513/? D/InputMethodManager: startInputInner - Id : 0
2020-01-22 17:04:00.674 513-513/? I/InputMethodManager: startInputInner - mService.startInputOrWindowGainedFocus
2020-01-22 17:04:00.685 513-539/? D/InputTransport: Input channel constructed: fd=71
2020-01-22 17:04:00.686 513-513/? D/InputMethodManager: prepareNavigationBarInfo() DecorView@ffa7d5a[MainActivity]
2020-01-22 17:04:00.686 513-513/? D/InputMethodManager: getNavigationBarColor() -855310
2020-01-22 17:04:00.686 513-513/? V/InputMethodManager: Starting input: tba=com.example.administracijazaposlenih ic=null mNaviBarColor -855310 mIsGetNaviBarColorSuccess true , NavVisible : true , NavTrans : false
2020-01-22 17:04:00.686 513-513/? D/InputMethodManager: startInputInner - Id : 0
2020-01-22 17:04:04.639 513-513/com.example.administracijazaposlenih D/ViewRootImpl@ec43217[MainActivity]: ViewPostIme pointer 0
2020-01-22 17:04:04.814 513-513/com.example.administracijazaposlenih D/ViewRootImpl@ec43217[MainActivity]: ViewPostIme pointer 1
2020-01-22 17:04:05.229 513-513/com.example.administracijazaposlenih D/ViewRootImpl@ec43217[MainActivity]: ViewPostIme pointer 0
2020-01-22 17:04:05.322 513-513/com.example.administracijazaposlenih D/ViewRootImpl@ec43217[MainActivity]: ViewPostIme pointer 1
2020-01-22 17:04:05.363 513-513/com.example.administracijazaposlenih D/InputMethodManager: prepareNavigationBarInfo() DecorView@ffa7d5a[MainActivity]
2020-01-22 17:04:05.363 513-513/com.example.administracijazaposlenih D/InputMethodManager: getNavigationBarColor() -855310
2020-01-22 17:04:05.363 513-513/com.example.administracijazaposlenih V/InputMethodManager: Starting input: tba=com.example.administracijazaposlenih ic=com.android.internal.widget.EditableInputConnection@6079d2e mNaviBarColor -855310 mIsGetNaviBarColorSuccess true , NavVisible : true , NavTrans : false
2020-01-22 17:04:05.364 513-513/com.example.administracijazaposlenih D/InputMethodManager: startInputInner - Id : 0
2020-01-22 17:04:05.373 513-513/com.example.administracijazaposlenih I/InputMethodManager: startInputInner - mService.startInputOrWindowGainedFocus
2020-01-22 17:04:05.379 513-513/com.example.administracijazaposlenih D/InputTransport: Input channel constructed: fd=75
2020-01-22 17:04:05.379 513-513/com.example.administracijazaposlenih D/InputTransport: Input channel destroyed: fd=71
2020-01-22 17:04:05.380 513-513/com.example.administracijazaposlenih D/InputMethodManager: SSI - flag : 0 Pid : 513 view : com.example.administracijazaposlenih
2020-01-22 17:04:05.381 513-513/com.example.administracijazaposlenih D/InputMethodManager: prepareNavigationBarInfo() DecorView@ffa7d5a[MainActivity]
2020-01-22 17:04:05.381 513-513/com.example.administracijazaposlenih D/InputMethodManager: getNavigationBarColor() -855310
2020-01-22 17:04:05.731 513-513/com.example.administracijazaposlenih D/ViewRootImpl@ec43217[MainActivity]: MSG_RESIZED: frame=Rect(0, 0 - 1440, 2960) ci=Rect(0, 96 - 0, 0) vi=Rect(0, 96 - 0, 1348) or=1
2020-01-22 17:04:09.055 513-513/com.example.administracijazaposlenih D/ViewRootImpl@ec43217[MainActivity]: ViewPostIme pointer 0
2020-01-22 17:04:09.098 513-513/com.example.administracijazaposlenih D/ViewRootImpl@ec43217[MainActivity]: ViewPostIme pointer 1
2020-01-22 17:04:09.141 513-513/com.example.administracijazaposlenih D/InputMethodManager: prepareNavigationBarInfo() DecorView@ffa7d5a[MainActivity]
2020-01-22 17:04:09.141 513-513/com.example.administracijazaposlenih D/InputMethodManager: getNavigationBarColor() -855310
2020-01-22 17:04:09.141 513-513/com.example.administracijazaposlenih V/InputMethodManager: Starting input: tba=com.example.administracijazaposlenih ic=com.android.internal.widget.EditableInputConnection@940c9ea mNaviBarColor -855310 mIsGetNaviBarColorSuccess true , NavVisible : true , NavTrans : false
2020-01-22 17:04:09.141 513-513/com.example.administracijazaposlenih D/InputMethodManager: startInputInner - Id : 0
2020-01-22 17:04:09.152 513-513/com.example.administracijazaposlenih I/InputMethodManager: startInputInner - mService.startInputOrWindowGainedFocus
2020-01-22 17:04:09.156 513-513/com.example.administracijazaposlenih D/InputTransport: Input channel constructed: fd=73
2020-01-22 17:04:09.156 513-513/com.example.administracijazaposlenih D/InputTransport: Input channel destroyed: fd=75
2020-01-22 17:04:09.156 513-513/com.example.administracijazaposlenih D/InputMethodManager: SSI - flag : 0 Pid : 513 view : com.example.administracijazaposlenih
2020-01-22 17:04:09.157 513-513/com.example.administracijazaposlenih D/InputMethodManager: prepareNavigationBarInfo() DecorView@ffa7d5a[MainActivity]
2020-01-22 17:04:09.157 513-513/com.example.administracijazaposlenih D/InputMethodManager: getNavigationBarColor() -855310
2020-01-22 17:04:09.181 513-513/com.example.administracijazaposlenih I/AssistStructure: Flattened final assist data: 3028 bytes, containing 1 windows, 10 views
2020-01-22 17:04:11.481 513-513/com.example.administracijazaposlenih D/ViewRootImpl@ec43217[MainActivity]: ViewPostIme key 0
2020-01-22 17:04:11.484 513-513/com.example.administracijazaposlenih D/ViewRootImpl@ec43217[MainActivity]: ViewPostIme key 1
2020-01-22 17:04:11.492 513-513/com.example.administracijazaposlenih D/InputMethodManager: HSIFW - flag : 0 Pid : 513
2020-01-22 17:04:11.581 513-513/com.example.administracijazaposlenih D/ViewRootImpl@ec43217[MainActivity]: MSG_RESIZED: frame=Rect(0, 0 - 1440, 2960) ci=Rect(0, 96 - 0, 0) vi=Rect(0, 96 - 0, 0) or=1
2020-01-22 17:04:12.067 513-513/com.example.administracijazaposlenih D/ViewRootImpl@ec43217[MainActivity]: ViewPostIme pointer 0
2020-01-22 17:04:12.105 513-513/com.example.administracijazaposlenih D/ViewRootImpl@ec43217[MainActivity]: ViewPostIme pointer 1
2020-01-22 17:04:12.195 513-513/com.example.administracijazaposlenih W/ActivityThread: handleWindowVisibility: no activity for token android.os.BinderProxy@9eae2a8
2020-01-22 17:04:12.214 513-513/com.example.administracijazaposlenih I/DecorView: createDecorCaptionView >> DecorView@e09d8fd[], isFloating: false, isApplication: true, hasWindowDecorCaption: false, hasWindowControllerCallback: true
2020-01-22 17:04:12.263 513-513/com.example.administracijazaposlenih D/AndroidRuntime: Shutting down VM
2020-01-22 17:04:12.265 513-513/com.example.administracijazaposlenih E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.administracijazaposlenih, PID: 513
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.administracijazaposlenih/com.example.administracijazaposlenih.Glavna}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3114)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3257)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1948)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7050)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:494)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:965)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at com.example.administracijazaposlenih.Glavna.onCreate(Glavna.java:21)
        at android.app.Activity.performCreate(Activity.java:7327)
        at android.app.Activity.performCreate(Activity.java:7318)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3094)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3257) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1948) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:214) 
        at android.app.ActivityThread.main(ActivityThread.java:7050) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:494) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:965) 
2020-01-22 17:04:12.295 513-513/com.example.administracijazaposlenih I/Process: Sending signal. PID: 513 SIG: 9

Sorry i am new in android studio, please help me, cheers:Dd

Answer 1

You are trying to call setOnClickListenener on a null object reference. When working with Java, make sure that ID you are passing to findViewById is exactly the same as it was declared in the xml layout else the variable instance will return a null object. Try putting a breakpoint on btn1.setOnClickListener... and investigate to see which of your buttons is null.

Answer 2

It show the Null Object reference. please check you button id into the Xml.

Answer 3

Kindly check your xml file code, make sure to define button and check button id with activity class id.

it error for you trying call to setOnClicklistener, but your button was null.

 Button button = (Button) findViewById(R.id.button);