java - 传递带有 Intent 的 Integer 会导致空对象引用

Question

我正在方法中创建一个新 Intent ，将整数值传递给另一个 Activity 。但是，我在接收端收到空对象引用/空指针异常。这是调用方法(Insect是自定义类型，ID为整数)

private void openInsectDetails(Insect insect){
        Intent intent = new Intent(InsectCatchable.this, InsectDetails.class);
        intent.putExtra("InsectDetails", insect.getId());
        InsectCatchable.this.startActivity(intent);
    }

这是接收类:

package com.ac_companion.wegner;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
import android.util.Log;

import java.util.ArrayList;

public class InsectDetails extends AppCompatActivity {
    private final String TAG = "InsectDetails";
    private int insectPositionMarker;
    private Insect localInsect;
    public int insectId;
    //private Intent mIntent = new Intent();

    public InsectDetails(){
        Intent mIntent = getIntent();
        insectId = mIntent.getIntExtra("InsectDetails", 0);
    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_insect_details);
        localInsect = Util.getGlobalInsectList().get(insectId);
    }

}

这是异常(exception):

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.ac_companion.wegner, PID: 16344
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.ac_companion.wegner/com.ac_companion.wegner.InsectDetails}: java.lang.NullPointerException: Attempt to invoke virtual method 'int android.content.Intent.getIntExtra(java.lang.String, int)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2548)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
        at android.app.ActivityThread.-wrap12(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6077)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int android.content.Intent.getIntExtra(java.lang.String, int)' on a null object reference
        at com.ac_companion.wegner.InsectDetails.<init>(InsectDetails.java:20)
        at java.lang.Class.newInstance(Native Method)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1078)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2538)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6077) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756) 

为什么我的mIntent.getIntExtra是一个空对象？据我了解，它已正确初始化。

Answer 1

我认为这是因为 Activity 生命周期。您不应该在 Activity 中使用构造函数。只需将您的代码移至 @OnCreate 方法

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

    Intent mIntent = getIntent();
    insectId = mIntent.getIntExtra("InsectDetails", 0);
    localInsect = Util.getGlobalInsectList().get(insectId);
}

您会遇到异常，因为您试图在创建 Activity 之前获取 Intent 。