1

我的java代码：

public class StartActivity extends AppCompatActivity {

    private Button mRegBtn;
    private Button mLoginBtn;

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

        mRegBtn = (Button) findViewById(R.id.start_reg_btn);
        mLoginBtn = (Button) findViewById(R.id.start_login_btn);

        mRegBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                Intent reg_intent = new Intent(StartActivity.this, RegisterActivity.class);
                startActivity(reg_intent);
            }
        });

        mLoginBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                Intent login_intent = new Intent(StartActivity.this, LoginActivity.class);
                startActivity(login_intent);

            }
        });
    }
}

LogCat 显示了这一点：

2020-04-30 12:53:42.391 31407-31407/com.example.chattting E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.chattting, PID: 31407
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.chattting/com.example.chattting.LoginActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2947)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3012)
        at android.app.ActivityThread.-wrap12(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1716)
        at android.os.Handler.dispatchMessage(Handler.java:110)
        at android.os.Looper.loop(Looper.java:232)
        at android.app.ActivityThread.main(ActivityThread.java:6802)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1103)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:964)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
        at com.example.chattting.LoginActivity.onCreate(LoginActivity.java:43)
        at android.app.Activity.performCreate(Activity.java:6974)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2900)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3012) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1716) 
        at android.os.Handler.dispatchMessage(Handler.java:110) 
        at android.os.Looper.loop(Looper.java:232) 
        at android.app.ActivityThread.main(ActivityThread.java:6802) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1103) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:964) 
4

1 回答 1

0

我给出的第一个答案。根据您的日志，“引起：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)'”行说，无法设置 AppBar 标题。我敢打赌，在您尝试从按钮开始的一项活动中，您有某种错误。如果您需要更多帮助，请告诉失败的按钮是什么，然后发送其他两个活动代码。

于 2020-04-30T09:00:29.200 回答