public class MainActivity extends AppCompatActivity {

    private LruCache<String, Bitmap> mMemoryCache;

    ImageView imageView;

    Bitmap bitmap;

    int cacheSize;

    @Override

    protected void onCreate(@Nullable Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        imageView = findViewById(R.id.image);

        long maxMemory = Runtime.getRuntime().maxMemory();

        cacheSize = (int) (maxMemory / 8)/1024;

        mMemoryCache = new LruCache<String, Bitmap>(

                cacheSize) {

            @Override

            protected int sizeOf(String key, Bitmap bitmap) {

                // 重写此方法来衡量每张图片的大小，默认返回图片数量。

                return bitmap.getRowBytes() * bitmap.getHeight() / 1024;

            }

            @Override

            protected void entryRemoved(boolean evicted, String key,

                                        Bitmap oldValue, Bitmap newValue) {

                Log.v("tag", "hard cache is full , push to soft cache");

            }

        };

    }

    @Override

    protected void onStart() {

        super.onStart();

        Bitmap bitmap2 = mMemoryCache.get("a");

        if (bitmap2 != null) {

            imageView.setImageBitmap(bitmap2);

        } else {

            bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.icon12);

            mMemoryCache.put("a",bitmap);

            imageView.setImageBitmap(bitmap);

        }

    }

}