java - Android 应用程序在 Intent 时崩溃

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What is a NullPointerException, and how do I fix it? (12 个回答)

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我是 Android 编程新手，现在我的应用程序停止了。 MainAcitivity用于登录，当单击“登录”按钮时，它应该更改为另一个 Activity ，但它崩溃了。这是我的 MainActivity 代码:

package com.example.mt;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.CheckBox;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
    private EditText accountEdit;
    private EditText passwordEdit;
    private Button login_button;
    private CheckBox rememberPass;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        final SharedPreferences settings = getPreferences(MODE_PRIVATE);
        accountEdit = findViewById(R.id.account);
        passwordEdit = findViewById(R.id.password);
        rememberPass = findViewById(R.id.remember_pass);
        login = findViewById(R.id.login);
        boolean isRemember = settings.getBoolean("remember_pass",false);
        if(isRemember){
            String account = settings.getString("account","");
            String password = settings.getString("password","");
            accountEdit.setText(account);
            passwordEdit.setText(password);
            rememberPass.setChecked(true);
        }
        login_button.setOnClickListener(new View.OnClickListener(){
            @Override
            public void onClick(View v){
                String account = accountEdit.getText().toString();
                String password = passwordEdit.getText().toString();
                if (account.equals("tianmiao")&&password.equals("tianmiao")){
                    SharedPreferences.Editor editor = settings.edit();
                    if(rememberPass.isChecked()){
                        editor.putBoolean("remember_password",true);
                        editor.putString("account",account);
                        editor.putString("password",password);
                    } else{
                        editor.clear();
                    }
                    editor.apply();
                    Intent intent = new Intent(MainActivity.this,SearchActivity.class);
                    startActivity(intent);
                    finish();
                }else{
                    Toast.makeText(MainActivity.this,"wrong account or password（both tianmiao）",Toast.LENGTH_SHORT).show();
                }
            }
        });
    }
}

这是 SearchAcitivity 代码:

package com.example.mt;

import androidx.appcompat.app.AppCompatActivity;

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

public class SearchActivity extends AppCompatActivity {
    private EditText searchEditText;
    private Button search_button;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        searchEditText = findViewById(R.id.search);
        search_button = findViewById(R.id.search_button);
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_search);
        search_button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
              String search = searchEditText.getText().toString();
              if(search.equals("balabala")){
                  Intent intent = new Intent(SearchActivity.this,ListActivity.class);
                  startActivity(intent);
                  finish();
              }else{
                  Toast.makeText(SearchActivity.this,"No"+search+"（there's only balabala）",Toast.LENGTH_SHORT).show();
              }

            }
        });
    }
}

主要 Activity xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    android:orientation="vertical"
    tools:context=".MainActivity">
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="60dp"
        android:orientation="horizontal">
    <TextView
        android:layout_width="90dp"
        android:layout_height="wrap_content"
        android:layout_gravity="center_vertical"
        android:textSize="18sp"
        android:text="account："/>
    <EditText
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        android:layout_gravity="center_vertical"
        android:id="@+id/account"/>

    </LinearLayout>
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="60dp"
        android:orientation="horizontal">
        <TextView
            android:layout_width="90dp"
            android:layout_height="wrap_content"
            android:layout_gravity="center_vertical"
            android:textSize="18sp"
            android:text="password："/>
        <EditText
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_gravity="center_vertical"
            android:layout_weight="1"
            android:id="@+id/password"
            android:inputType="textPassword"/>
    </LinearLayout>
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">
        <CheckBox
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/remember_pass"/>
        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textSize="18sp"
            android:text="remember"/>
    </LinearLayout>
    <Button
        android:layout_width="wrap_content"
        android:layout_height="60dp"
        android:id="@+id/login_button"
        android:text="login"/>
    </LinearLayout>

搜索 Activity xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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=".SearchActivity"
    android:orientation="vertical">
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="60dp"
        android:orientation="horizontal">
        <TextView
            android:layout_width="90dp"
            android:layout_height="wrap_content"
            android:layout_gravity="center_vertical"
            android:textSize="18sp"
            android:text="buy what?："/>
        <EditText
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:layout_gravity="center_vertical"
            android:id="@+id/search"/>

    </LinearLayout>
    <Button
        android:layout_width="wrap_content"
        android:layout_height="60dp"
        android:id="@+id/search_button"
        android:text="search"/>

</LinearLayout>

这是 list :

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.mt">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".SearchActivity"></activity>
        <activity android:name=".ListActivity" />
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

    </application>

</manifest>

logcat 的红色部分:

2019-12-22 19:10:30.600 12135-12135/? E/com.example.mt: Unknown bits set in runtime_flags: 0x8000
2019-12-22 19:10:31.403 12135-12161/com.example.mt E/eglCodecCommon: glUtilsParamSize: unknow param 0x000082da
2019-12-22 19:10:31.404 12135-12161/com.example.mt E/eglCodecCommon: glUtilsParamSize: unknow param 0x000082da
Process: com.example.mt, PID: 12135
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.mt/com.example.mt.SearchActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$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$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at com.example.mt.SearchActivity.onCreate(SearchActivity.java:21)
        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) 
2019-12-22 19:10:49.884 12135-12135/com.example.mt I/Process: Sending signal. PID: 12135 SIG: 9

谢谢!

Answer 1

在 layout 膨胀之前，您正在将 views 与 ids 连接，这会在此处给您带来 空指针异常 :

    searchEditText = findViewById(R.id.search);
    search_button = findViewById(R.id.search_button);
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_search);

您需要移动:

    searchEditText = findViewById(R.id.search);
    search_button = findViewById(R.id.search_button);

下面:setContentView(R.layout.activity_search);像这样:

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_search);

    //now view is inflated you can connect views with ids
    searchEditText = findViewById(R.id.search);
    search_button = findViewById(R.id.search_button);