上一个按钮进行3张图片切换，下一个按钮进行2张图片切换

[YXTS122]

为什么反复点击上一个按钮进行3张图片切换，为什么反复点击下一个按钮进行2张图片切换？？？？？

public class MainActivity extends Activity {
	 ImageView img;
	    Button btn_last,btn_next;

	    private int[] imgs={
	            R.drawable.abc_ab_bottom_solid_dark_holo,
	            R.drawable.abc_ic_voice_search,
	            R.drawable.ic_launcher,
	            R.drawable.abc_textfield_search_default_holo_light,
	    };
	    int index=1;
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		 btn_last=(Button)findViewById(R.id.btn_last);
	        btn_next=(Button)findViewById(R.id.btn_next);
	        img=(ImageView)findViewById(R.id.image);
	        btn_last.setOnClickListener(new mClick());
	        btn_next.setOnClickListener(new mClick());

	}
	 class  mClick implements  OnClickListener
	    {
	        public void onClick(View v)
	        {
	            if(v==btn_last)
	            {
	                if(index>0&&index<imgs.length)
	                {
	                    index--;
	                    img.setImageResource(imgs[index]);
	                }else {index=imgs.length-1;}
	            }
	            if(v==btn_next)
	            {
	                if(index>0&&index<imgs.length-1)
	                {
	                    index++;
	                    img.setImageResource(imgs[index]);
	                }else {index=1;}
	            }
	        }
	    }
}  

[顽固已死_有事烧纸]

if(v==btn_last)
                {
                    if(index>0&&index<imgs.length)
                    {
                        index--;
                       
                    }else {
                            index=imgs.length - 1;
                    }
                    img.setImageResource(imgs[index]);
                }
                if(v==btn_next)
                {
                    if(index>=0&&index<imgs.length-1)
                    {
                        index++;
                        
                    }else {index=0;}
                   img.setImageResource(imgs[index]);
                }