java - E/Android运行时: FATAL EXCEPTION: main error in android studio

Question

我的应用程序是一个简单的计数器，带有添加按钮和重置按钮。当我尝试在模拟器上运行我的应用程序时，总是需要更多时间来安装并最终显示此错误消息。

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.tasbeeh, PID: 7966
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.tasbeeh/com.example.tasbeeh.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.widget.TextView.findViewById(int)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.widget.TextView.findViewById(int)' on a null object reference
        at com.example.tasbeeh.MainActivity.onCreate(MainActivity.java:24)
        at android.app.Activity.performCreate(Activity.java:7802)
        at android.app.Activity.performCreate(Activity.java:7791)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1299)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) 
        at android.os.Handler.dispatchMessage(Handler.java:107) 
        at android.os.Looper.loop(Looper.java:214) 
        at android.app.ActivityThread.main(ActivityThread.java:7356) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)

这是我的 MainActivity.java

package com.example.tasbeeh;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    TextView tvCounter;
    Button add_button;
    ImageView resetButton;


    int counter = 0;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        tvCounter.findViewById(R.id.tvCounter);
        add_button.findViewById(R.id.add_button);
        resetButton.findViewById(R.id.resetButton);


        add_button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                if(counter == 0) {
                    counter = counter + 1;
                    tvCounter.setText(String.valueOf(counter));
                }
            }
        });

        resetButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                counter = 0;
            }
        });
    }
}

Answer 1

findViewById() 应该用作:

tvCounter = findViewById(R.id.tvCounter);
add_button = findViewById(R.id.add_button);
resetButton = findViewById(R.id.resetButton);