在 android 中使用多屏时应用程序崩溃

Question

我正在尝试在 android 工作室中构建一个多屏应用程序，但不幸的是，它在运行期间崩溃了。 该应用程序在主屏幕上运行良好，它不会在MainActivity上崩溃，但在MainActivity2上它会崩溃。

这是我的 MainActivity 的 Java 代码：

package com.example.scrollapp;

import androidx.appcompat.app.AppCompatActivity;    
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
    EditText editText;
    public static final String EXTRA_NAME ="com.example.scrollapp.extra.NAME";    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        editText=findViewById(R.id.editTextTextPersonName3);
        String nametext = editText.getText().toString();    
    }
    public void openActivity(View v){
    Toast.makeText(MainActivity.this, "opening second activity", Toast.LENGTH_SHORT).show();
        Intent intent = new Intent(this,MainActivity2.class);
        editText=findViewById(R.id.editTextTextPersonName3);
        String nametext = editText.getText().toString();
        intent.putExtra(EXTRA_NAME,nametext);
        startActivity(intent);
    }        
}

这是 MainActivity2 的 Java 代码：

package com.example.scrollapp;    

import androidx.appcompat.app.AppCompatActivity;    
import android.annotation.SuppressLint;
import android.content.Intent;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity2 extends AppCompatActivity {
    TextView textView;
    @SuppressLint("SetTextI18n")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        textView.findViewById(R.id.textView);
        Intent intent = getIntent();
        String name = intent.getStringExtra(MainActivity.EXTRA_NAME);
        textView.setText("your name is "+ name);
    }
}

logcat中显示的错误：

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.scrollapp, PID: 7978
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.scrollapp/com.example.scrollapp.MainActivity2}: 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:2325)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
    at android.app.ActivityThread.access$800(ActivityThread.java:151)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5254)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
 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.scrollapp.MainActivity2.onCreate(MainActivity2.java:18)
    at android.app.Activity.performCreate(Activity.java:5990)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387) 
    at android.app.ActivityThread.access$800(ActivityThread.java:151) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:135) 
    at android.app.ActivityThread.main(ActivityThread.java:5254) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at java.lang.reflect.Method.invoke(Method.java:372) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698) 
I/Process: Sending signal. PID: 7978 SIG: 9

Answer 1

MainActivity2 class 中的 TextView textview 未初始化。 使用textView = findViewById(R.id.textView); 而不是textView.findViewById(R.id.textView); 在 MainActivity2 class。

Answer 2

TextView的初始化不正确。 正确的初始化方式是：

textView = findViewById(R.id.textView);

您必须在调用setcontentView方法后实例化变量，因此您必须在MainActivity2中执行以下操作：

public class MainActivity2 extends AppCompatActivity {
    TextView textView;
    @SuppressLint("SetTextI18n")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        textView = findViewById(R.id.textView);// Change made here in this line
        Intent intent = getIntent();
        String name = intent.getStringExtra(MainActivity.EXTRA_NAME);
        textView.setText("your name is "+ name)
    }
}