如何从另一个类调用MainActivity中的方法?

How do I call the method startChronometer in another class when the method is declared inside main activity?

当在主活动中声明方法时，如何调用另一个类中的方法startChronometer ?

Inside MainActivity:

在MainActivity:

public void startChronometer(){
        mChronometer.start();
        showElapsedTime();
    }

Inside another class, I tried to do this:

在另一个课堂上，我试着这样做:

MainActivity mActivity;
mActivity.startChronometer();

But an error occurred which says

但是发生了一个错误

java.lang.NullPointerException.

java.lang.NullPointerException。

May I know what more do I need to add to the code?

我可以知道我还需要添加什么到代码中吗?

10 个解决方案

#1

You can easily call a method from any Fragment inside your Activity by just doing a cast like this:

您可以通过以下方式从活动中的任何片段调用方法:

((MainActivity)getActivity()).startChronometer();

Hope this helps!

希望这可以帮助!

#2

Initialize it first

首先初始化

MainActivity mActivity= new MainActivity();

Then you can continue

然后你可以继续

mActivity.startChronometer();

#3

I would suggest, one should not make object of an Activity type class.

我的建议是，不应该创建活动类型类的对象。

MainActivity mActivity = new MainActivity();  // BIG NO TO THIS.

All Activities in Android must go through the Activity lifecycle so that they have a valid context attached to them.

Android中的所有活动都必须经过活动生命周期，以便它们有一个有效的上下文。

By treating an Activity as a normal Java class, you end up with a null context. As most methods in an Activity are called on its Context, you will get a null pointer exception, which is why your app crashes.

通过将活动视为一个普通的Java类，您将得到一个空上下文。由于活动中的大多数方法在其上下文中被调用，您将得到一个空指针异常，这就是为什么您的应用程序会崩溃。

Instead, move all such methods which need to be called from other classes into a Utility class which accepts a valid context in its constructor, and then use that context in the methods to do the work.

相反，将需要从其他类调用的所有此类方法移动到实用程序类中，该实用程序类在其构造函数中接受有效上下文，然后在方法中使用该上下文来完成工作。

#4

MainActivity.java

MainActivity.java

public class MainActivity extends AppCompatActivity {

    private static MainActivity instance;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        instance = this;
    }

    public static MainActivity getInstance() {
        return instance;
    }

    public void myMethod() {
       // do something...
    }
)

AnotherClass.java

AnotherClass.java

public Class AnotherClass() {
     // call this method
     MainActivity.getInstance().myMethod();
}

#5

But an error occurred which says java.lang.NullPointerException.

但是出现了一个错误，它说java.lang.NullPointerException。

Thats because, you never initialized your MainActivity. you should initialize your object before you call its methods.

这是因为，您从未初始化主活动。在调用它的方法之前，应该初始化对象。

MainActivity mActivity = new MainActivity();//make sure that you pass the appropriate arguments if you have an args constructor
mActivity.startChronometer();

#6

Use this code in sub Fragment of MainActivity to call the method on it.

在MainActivity的子片段中使用此代码调用该方法。

((MainActivity) getActivity()).startChronometer();

#7

You have to pass instance of MainActivity into another class, then you can call everything public (in MainActivity) from everywhere.

您必须将MainActivity实例传递到另一个类中，然后您可以从任何地方调用everything public(在MainActivity中)。

MainActivity.java

MainActivity.java

public class MainActivity extends AppCompatActivity {

    // Instance of AnotherClass for future use
    private AnotherClass anotherClass;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // Create new instance of AnotherClass and
        // pass instance of MainActivity by "this"
        anotherClass = new AnotherClass(this);
    }

    // Method you want to call from another class
    public void startChronometer(){
        ...
    }
}

AnotherClass.java

AnotherClass.java

public class AnotherClass {

    // Main class instance
    private MainActivity mainActivity;  

    // Constructor
    public AnotherClass(MainActivity activity) {

        // Save instance of main class for future use
        mainActivity = activity;  

        // Call method in MainActivity
        mainActivity.startChronometer();
    }
}

#8

What i have done and it works is create an instance in the MainActivity and getter for that instance:

我所做的和它的工作是在MainActivity和getter中创建一个实例:

 public class MainActivity extends AbstractMainActivity {
    private static MainActivity mInstanceActivity;
    public static MainActivity getmInstanceActivity() {
    return mInstanceActivity;
    }

And the in the onCreate method just point to that activity:

onCreate方法中只指向这个活动

@Override
protected void onCreate(Bundle savedInstanceState) {
mInstanceActivity = this;
}

And in onDestroy you should set this instance to null:

在onDestroy中，你应该将这个实例设置为null:

@Override
protected void onDestroy() {
    super.onDestroy();
    mInstanceActivity = null;
}

Later you can call every method in whatever class you want:

之后你可以调用任何你想要的类中的每个方法:

MainActivity.getmInstanceActivity().yourMethod();

#9

Simply, You can make this method static as below:

简单地说，您可以使这个方法保持静态，如下所示:

public static void startChronometer(){
    mChronometer.start();
    showElapsedTime();
} 

you can call this function in other class as below:

可以在其他类中调用此函数，如下所示:

MainActivity.startChronometer();

#10

You can make this method static.

您可以使这个方法静态。

public static void startChronometer(){
        mChronometer.start();
        showElapsedTime();
    } 

you can call this function in other class as below:

可以在其他类中调用此函数，如下所示:

MainActivity.startChronometer();

OR

或

You can make an object of the main class in second class like,

你可以在第二类中创建一个主类的对象，

MainActivity mActivity = new MainActivity();
mActivity.startChronometer();