Android空指针异常：Button.setOnClickListener调用空对象引用问题

问题根源 #

你触发空指针的核心原因是调用findViewById的时机早于setContentView。在setContentView执行前，布局还未加载到内存中，所有通过findViewById获取的控件实例都是null，此时调用setOnClickListener必然抛出空指针异常。

修复方案 #

将所有控件初始化（findViewById）的代码，移动到setContentView(R.layout.activity_main)之后执行。修改后的onCreate方法如下：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    EdgeToEdge.enable(this);
    setContentView(R.layout.activity_main);
    
    // 现在才初始化控件，此时布局已加载
    sB = findViewById(R.id.selectB);
    pB = findViewById(R.id.PredicteB);
    cB = findViewById(R.id.captureB);
    result = findViewById(R.id.result);
    imageView = findViewById(R.id.imageView);

    ViewCompat.setOnApplyWindowInsetsListener(findViewById(R.id.main), (v, insets) -> {
        Insets systemBars = insets.getInsets(WindowInsetsCompat.Type.systemBars());
        v.setPadding(systemBars.left, systemBars.top, systemBars.right, systemBars.bottom);
        return insets;
    });

    // 后续的点击监听设置逻辑保持不变
    sB.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent intent1 = new Intent();
            intent1.setAction(Intent.ACTION_GET_CONTENT);
            intent1.setType("image/*");
            startActivityForResult(intent1,10);
        }
    });

    cB.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent intent2 = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
            startActivityForResult(intent2, 12);
        }
    });

    getPermission();
    int cnt = 0;

    String[] lables = new String[1001];

    try {
        BufferedReader br= new BufferedReader(new InputStreamReader(getAssets().open("labels.txt")));
        String line = br.readLine();
        while (line != null){
            lables[cnt] = line;
            cnt++;
            line = br.readLine();
        }
    } catch (IOException e) {
        e.printStackTrace();
    }

    pB.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            // 新增：先判断Bitmap是否为空，避免崩溃
            if (bm == null) {
                result.setText("请先选择或拍摄图片");
                return;
            }
            
            try {
                MobilenetV110224Quant model = MobilenetV110224Quant.newInstance(MainActivity.this);

                TensorBuffer inputFeature0 = TensorBuffer.createFixedSize(new int[]{1, 224, 224, 3}, DataType.UINT8);
                Bitmap scaledBm = Bitmap.createScaledBitmap(bm, 224, 224, true);
                inputFeature0.loadBuffer(TensorImage.fromBitmap(scaledBm).getBuffer());

                MobilenetV110224Quant.Outputs outputs = model.process(inputFeature0);
                TensorBuffer outputFeature0 = outputs.getOutputFeature0AsTensorBuffer();

                result.setText(lables[getMax(outputFeature0.getFloatArray())]);

                model.close();
            } catch (IOException e) {
                e.printStackTrace();
                result.setText("识别失败");
            }
        }
    });
}

额外优化建议 #

1. 相机拍照结果处理：在onActivityResult的相机分支（requestCode=12）中，增加data非空判断，避免空指针：

else if(requestCode == 12){
    if (data != null) {
        bm = (Bitmap) data.getExtras().get("data");
        imageView.setImageBitmap(bm);
    }
}

2. 权限请求逻辑：当前权限请求如果用户拒绝后会无限弹窗，可添加提示引导用户开启权限：

void getPermission(){
    if(checkSelfPermission(Manifest.permission.CAMERA) != PackageManager.PERMISSION_GRANTED){
        if (ActivityCompat.shouldShowRequestPermissionRationale(this, Manifest.permission.CAMERA)) {
            Toast.makeText(this, "需要相机权限来拍摄图片", Toast.LENGTH_SHORT).show();
        }
        ActivityCompat.requestPermissions(MainActivity.this,new String[]{Manifest.permission.CAMERA},11);
    }
}

内容的提问来源于stack exchange，提问作者Nier Yorha