Java getResources（）.getDisplayMetrics（）上的NullPointerException。密度 上下文

我现在正在编写一个小的测试代码。它选择图像，根据屏幕密度调整图像大小以适合屏幕。当它执行此操作时，它将按照图像在

xxhdpi

显示中的大小复制图像，并将其转换为位图，然后转换为字符串。然后，该字符串将通过一个Intent进入下一个屏幕，在该屏幕中，该字符串被转换回位图，然后放置在

ImageButton

中

这很好，直到我添加了一段代码，根据密度调整字符串给出的图像大小。奇怪的是，我从另一个活动复制并粘贴了DPI（）方法，而该活动中没有任何问题

密码 错误日志

01-15 19:55:17.104 30570-30570/com.example.zachary.imagetesting D/ViewRootImpl:ViewPostImeInputStage操作
01-15 19:55:17.194 30570-30570/com.example.zachary.imagetesting E/位图字符串：/9j/4aaqskzjrgabaqaaqaabad/2wbdaaaebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqeba
qebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqebaqeb
AQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEB
AhEBAxEB/8QAHWAAAQUBAQEBAQEAAAAAAAAECAWQFBGCICQOL/8QAtRAAAgEDAwIEAwUFBAQA
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3/DFN69FN2IVTP2HPITCGHVFFxHshXGERA2SalkourtonWMergyimyHPJGDXWW/sG6gV/0j4oW
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1BEMT72D3Hgyw1EH/b3NHy+0lf+vU+SAK9G8VFCH4OECXLHIXWPR9HDCCxV47F9Q0KCRU/tbTD
e6eBxnDXQbGMgE8+CJB5BYPUEXX6NV8ARXKNMVOMPICNI6AQ4XeCdRvKfWnVpto9RTPylHxld
tfn33jKM1eEoyXeLUl16pvt+ers7lfqF8ItCX43/ALKY8H6y8VxqGnHV9A0bULtRI2n6no85vPDV
package com.example.zachary.imagetesting.Resizing;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.util.Base64;
import android.view.View;
import android.widget.ImageButton;
import android.widget.TextView;

import com.example.zachary.imagetesting.R;

public class PictureDecoded extends Activity {

    TextView Text;
    ImageButton Image;
    String string;
    Bitmap picture;
    public float DPI(){return getResources().getDisplayMetrics().density;}
    float dpi = DPI() * 180;
    int size = (int) dpi;


    public Bitmap Pic(String string){
        try{
            byte[] b = Base64.decode(string, Base64.DEFAULT);
            Bitmap bitmap = BitmapFactory.decodeByteArray(b, 0, b.length);
            return bitmap;
        } catch (Exception e) {
            e.getMessage();
            return null;
        }
    }

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.resizing);
        string = getIntent().getStringExtra("String");

        Image = (ImageButton) findViewById(R.id.image);
        Text = (TextView) findViewById(R.id.text);

        picture = Pic(string);

        Image.setImageBitmap(Bitmap.createScaledBitmap(picture,size,size,false));
    }

    public void next(View view){ }

    // I also tried addring DPI() here as well to no success.
    //public float DPI(){return getResources().getDisplayMetrics().density;}

}

01-15 19:55:17.104 30570-30570/com.example.zachary.imagetesting D/ViewRootImpl: ViewPostImeInputStage ACTION_DOWN
01-15 19:55:17.194 30570-30570/com.example.zachary.imagetesting E/Bitmap String:: /9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEB
                                                                                  AQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQH/2wBDAQEBAQEBAQEBAQEBAQEBAQEBAQEB
                                                                                  AQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQH/wAARCAGVAhwDASIA
                                                                                  AhEBAxEB/8QAHwAAAQUBAQEBAQEAAAAAAAAAAAECAwQFBgcICQoL/8QAtRAAAgEDAwIEAwUFBAQA
                                                                                  AAF9AQIDAAQRBRIhMUEGE1FhByJxFDKBkaEII0KxwRVS0fAkM2JyggkKFhcYGRolJicoKSo0NTY3
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                                                                                  AwEBAQEBAQEBAQAAAAAAAAECAwQFBgcICQoL/8QAtREAAgECBAQDBAcFBAQAAQJ3AAECAxEEBSEx
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                                                                                  oooAKKKKACiiigAooooAKKKKACiiigAooooAKKKKACiiigAooooAKKKKACiiigAooooAKKKKACii
                                                                                  igAooooAKKKKACiiigAooooAKKKKACiiigAooooAKKKKACiiigAooooAKKKKACiiigAooooAKKKK
                                                                                  ACiiigAooooAKKKKACiiigAooooAKKKKACiiigAooooAKKKKACiiigAooooAKKKKACiiigAooooA
                                                                                  KKKKACiiigAooooAKKKKACiiigA45GemM+3XGeTjOOPx5ODkruPAuv6DpWppYeMtHj1zwhqk0MOt
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01-15 19:55:17.194 30570-30570/com.example.zachary.imagetesting I/Timeline: Timeline: Activity_launch_request id:com.example.zachary.imagetesting time:50420468
01-15 19:55:17.244 30570-30570/com.example.zachary.imagetesting D/AndroidRuntime: Shutting down VM
01-15 19:55:17.244 30570-30570/com.example.zachary.imagetesting E/AndroidRuntime: FATAL EXCEPTION: main
                                                                                  Process: com.example.zachary.imagetesting, PID: 30570
                                                                                  java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.zachary.imagetesting/com.example.zachary.imagetesting.Resizing.PictureDecoded}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
                                                                                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2546)
                                                                                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2758)
                                                                                      at android.app.ActivityThread.access$900(ActivityThread.java:177)
                                                                                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1448)
                                                                                      at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                                      at android.os.Looper.loop(Looper.java:145)
                                                                                      at android.app.ActivityThread.main(ActivityThread.java:5942)
                                                                                      at java.lang.reflect.Method.invoke(Native Method)
                                                                                      at java.lang.reflect.Method.invoke(Method.java:372)
                                                                                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
                                                                                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)
                                                                                   Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
                                                                                      at android.content.ContextWrapper.getResources(ContextWrapper.java:90)
                                                                                      at android.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:74)
                                                                                      at com.example.zachary.imagetesting.Resizing.PictureDecoded.DPI(PictureDecoded.java:20)
                                                                                      at com.example.zachary.imagetesting.Resizing.PictureDecoded.<init>(PictureDecoded.java:21)
                                                                                      at java.lang.reflect.Constructor.newInstance(Native Method)
                                                                                      at java.lang.Class.newInstance(Class.java:1650)
                                                                                      at android.app.Instrumentation.newActivity(Instrumentation.java:1079)
                                                                                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2536)
                                                                                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2758) 
                                                                                      at android.app.ActivityThread.access$900(ActivityThread.java:177) 
                                                                                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1448) 
                                                                                      at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                                      at android.os.Looper.loop(Looper.java:145) 
                                                                                      at android.app.ActivityThread.main(ActivityThread.java:5942) 
                                                                                      at java.lang.reflect.Method.invoke(Native Method) 
                                                                                      at java.lang.reflect.Method.invoke(Method.java:372) 
                                                                                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399) 
                                                                                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)