java - 必须单击两次才能获取上一个数组元素

Question

我想制作一个带有上一个和下一个按钮的图库应用。我制作了照片的数组。每当我到达最后一张照片时，我必须单击两次上一个按钮才能获取上一张照片。而且当我到达第一张照片时，我必须单击两次下一步按钮才能到达下一张照片。我的代码:

public class MainActivity extends AppCompatActivity {

    ImageView ivphoto;
    Button btnext;
    Button btprevious;
    int a=0;
    int photoarray[]={R.drawable.cat, R.drawable.dog, R.drawable.duck, R.drawable.elephant, R.drawable.monkey, R.drawable.pig, R.drawable.rabbit, R.drawable.tiger};

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

        ivphoto = findViewById(R.id.ivphoto);
        btnext = findViewById(R.id.btnext);
        btprevious = findViewById(R.id.btprevious);

        btnext.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                ivphoto.setImageResource(photoarray[a]);
                a++;
                if (a==8){
                    a=7;
                    Toast.makeText(MainActivity.this, "This is last photo", Toast.LENGTH_SHORT).show();
                }
            }
        });

        btprevious.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                ivphoto.setImageResource(photoarray[a]);
                a--;
                if(a==-1){
                    a=0;
                    Toast.makeText(MainActivity.this, "This is first photo.", Toast.LENGTH_SHORT).show();
                }
            }
        });
    }
}

任何人都请帮我解决这个问题。谢谢。

Answer 1

在下一个按钮中尝试此代码:

if(a == photoarray.lenght - 1)
    Toast.makeText(MainActivity.this, "This is last photo", Toast.LENGTH_SHORT).show();
else
    ivphoto.setImageResource(photoarray[++a]);

后退按钮中的这段代码:

if(a == 0)
    Toast.makeText(MainActivity.this, "This is first photo.", Toast.LENGTH_SHORT).show();
else
    ivphoto.setImageResource(photoarray[--a]);