我是Android新手，只是想弄清楚如何使用onActivityResult方法。我想单击mainActivity上的按钮，转到另一个actvivity，输入名称并返回mainActivity。但是我在运行应用程序时遇到了麻烦。我总是得到这个错误：使用意图不能调用其他活动
java.lang.NullPointerException: Attempt to invoke virtual method...
09-20 22:14:32.212 30617-30617/com.genaepic.p029_simpleactivityresult E/AndroidRuntime:FATAL EXCEPTION: main Process: com.genaepic.p029_simpleactivityresult, PID: 30617 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.genaepic.p029_simpleactivityresult/com.genaepic.p029_simpleactivityresult.NameActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2665) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) at android.app.ActivityThread.-wrap12(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:154) at android.app.ActivityThread.main(ActivityThread.java:6119) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference at com.genaepic.p029_simpleactivityresult.NameActivity.onCreate(NameActivity.java:22) at android.app.Activity.performCreate(Activity.java:6679) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2618) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) at android.app.ActivityThread.-wrap12(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:154) at android.app.ActivityThread.main(ActivityThread.java:6119) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
我的主要活动：
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
TextView textView;
Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
textView = (TextView) findViewById(R.id.tv_main);
button = (Button) findViewById(R.id.btn_iputName);
button.setOnClickListener(this);
}
@Override
public void onClick(View v) {
Intent intent = new Intent(this, NameActivity.class);
startActivityForResult(intent, 1);
}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);
if(data == null) {
return;
}
String name = data.getStringExtra("name");
textView.setText("Your name is " + name);
}
}
我的第二个活动：为mainActivity
public class NameActivity extends AppCompatActivity implements View.OnClickListener {
EditText editText;
Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_name);
editText = (EditText) findViewById(R.id.et_name);
button = (Button) findViewById(R.id.btn_iputName);
button.setOnClickListener(this);
}
@Override
public void onClick(View v) {
Intent intent = new Intent();
intent.putExtra("name", editText.getText().toString());
setResult(RESULT_OK, intent);
finish();
}
}
我的XML代码：
<Button
android:id="@+id/btn_iputName"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="input your name"
tools:layout_constraintTop_creator="1"
android:layout_marginStart="34dp"
android:layout_marginTop="92dp"
tools:layout_constraintLeft_creator="1"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintTop_toTopOf="parent"
android:layout_marginLeft="34dp" />
<TextView
android:id="@+id/tv_main"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Your name is"
tools:layout_constraintTop_creator="1"
android:layout_marginStart="34dp"
android:layout_marginTop="33dp"
tools:layout_constraintLeft_creator="1"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintTop_toTopOf="parent"
android:layout_marginLeft="34dp" />
为NameActivity
<Button
android:id="@+id/btn_ok"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="116dp"
android:text="ok"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
tools:layout_constraintLeft_creator="1"
tools:layout_constraintRight_creator="1"
tools:layout_constraintTop_creator="1" />
<EditText
android:id="@+id/et_name"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginEnd="64dp"
android:layout_marginRight="84dp"
android:layout_marginTop="31dp"
android:ems="10"
android:inputType="textPersonName"
android:text="Name"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
tools:layout_constraintRight_creator="1"
tools:layout_constraintTop_creator="1" />
</android.support.constraint.ConstraintLayout>
您可以使用其中之一旁边的复选标记接受答案 –