android studio错误日志这样的 不知道哪里错误 郁闷哭了

01-22 01:23:35.234 25771-25771/com.example.dell_user.wechat E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.dell_user.wechat, PID: 25771
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.dell_user.wechat/com.example.dell_user.wechat.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.ViewGroup$LayoutParams android.widget.ImageView.getLayoutParams()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2703)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2774)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1588)
at android.os.Handler.dispatchMessage(Handler.java:111)
at android.os.Looper.loop(Looper.java:207)
at android.app.ActivityThread.main(ActivityThread.java:5955)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:930)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:791)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.ViewGroup$LayoutParams android.widget.ImageView.getLayoutParams()' on a null object reference
at com.example.dell_user.wechat.MainActivity.initLineImage(MainActivity.java:93)
at com.example.dell_user.wechat.MainActivity.onCreate(MainActivity.java:40)
at android.app.Activity.performCreate(Activity.java:6378)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1124)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2656)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2774) 
at android.app.ActivityThread.-wrap11(ActivityThread.java) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1588) 
at android.os.Handler.dispatchMessage(Handler.java:111) 
at android.os.Looper.loop(Looper.java:207) 
at android.app.ActivityThread.main(ActivityThread.java:5955) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:930) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:791) 

at com.example.dell_user.wechat.MainActivity.initLineImage(MainActivity.java:93)

使用了null空对象。导致异常了。看看这行代码

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.ViewGroup$LayoutParams android.widget.ImageView.getLayoutParams()' on a null object reference
at com.example.dell_user.wechat.MainActivity.initLineImage(MainActivity.java:93)
at com.example.dell_user.wechat.MainActivity.onCreate(MainActivity.java:40)
第93行有问题，仔细检查一下

public class MainActivity extends FragmentActivity implements ViewPager.OnPageChangeListener,View.OnClickListener {
ViewPager viewPager;
PagerAdapter pagerAdapter;
private List titleList;
private TextView tv1,tv2,tv3,tv4;
private ImageView tab;
private int moveOne =0;
private boolean isScrolling=false;
private boolean isBackScolling=false; //手指离开后的回弹
private long starTime =0;
private long currentTime =0 ;
Listfrag =new ArrayList<>();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    viewPager= (ViewPager) findViewById(R.id.viewpage);
    initLineImage();
    frag.add(new fragment());
    frag.add(new fragment2());
    frag.add(new fragment3());
    frag.add(new fragment4());
    pagerAdapter=new MyPagerAdapter(getSupportFragmentManager(),frag) ;
    viewPager.setAdapter(pagerAdapter);



    tv1= (TextView) findViewById(R.id.tv1);
    tv2= (TextView) findViewById(R.id.tv2);
    tv3= (TextView) findViewById(R.id.tv3);
    tv4= (TextView) findViewById(R.id.tv4);

    viewPager.setCurrentItem(0);
    tv1.setTextColor(Color.RED);
    tv2.setTextColor(Color.GREEN);
    tv3.setTextColor(Color.BLACK);
    tv4.setTextColor(Color.BLACK);
    viewPager.addOnPageChangeListener(MainActivity.this);
    tab= (ImageView) findViewById(R.id.tab);




}



private void movePositionX(int toPostion ,float positionOffestPiels) {           //下划线跟随手指的滑动而移动
    float  curTranslationX=tab.getTranslationX();                                     //当钱的translationX是的到滑块的位置
    float toPostionX=moveOne*toPostion  + positionOffestPiels;                        //
    ObjectAnimator animator=ObjectAnimator. ofFloat(tab,"translationX",curTranslationX,toPostionX);
    animator.setDuration(500);
    animator.start();
}



private void movePositionX(int toPostion){
    movePositionX(toPostion,0);
}



private void initLineImage(){
    DisplayMetrics dm = new DisplayMetrics();
    getWindowManager().getDefaultDisplay().getMetrics(dm);
    int screenw = dm.widthPixels;
    LayoutParams lp = tab.getLayoutParams();
    lp.width = screenw / 4;
    tab.setLayoutParams(lp);
    moveOne = lp.width;
}

@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) { //position是偏移量的百分比，positionOffsetPixel是偏移量
    currentTime=System.currentTimeMillis();
    if (isScrolling&&(currentTime-starTime)>200){
        movePositionX(position,moveOne*positionOffset);      //moveone 是滑动到第一个界面的距离
        starTime=currentTime;

    }
    if (isBackScolling){
        movePositionX(position);
    }
}

@Override
public void onPageSelected(int position) {                       //当界面转换时调用该方法
    switch (position) {
        case 0:
            tv1.setTextColor(Color.BLACK);
            tv2.setTextColor(Color.RED);
            tv3.setTextColor(Color.GREEN);
            tv4.setTextColor(Color.GRAY);
            movePositionX(0);
            break;
        case 1:
            tv1.setTextColor(Color.RED);
            tv2.setTextColor(Color.BLACK);
            tv3.setTextColor(Color.BLUE);
            tv4.setTextColor(Color.BLACK);
            movePositionX(1);
            break;
        case 2:
            tv1.setTextColor(Color.BLACK);
            tv2.setTextColor(Color.BLACK);
            tv3.setTextColor(Color.BLUE);
            tv4.setTextColor(Color.BLACK);
            movePositionX(2);
            break;
        case 3:
            tv1.setTextColor(Color.BLACK);
            tv2.setTextColor(Color.BLACK);
            tv3.setTextColor(Color.GRAY);
            tv4.setTextColor(Color.BLACK);
            movePositionX(3);
            break;

        default:
            break;


    }
}

@Override
public void onPageScrollStateChanged(int state) {       //判断手指什么时候滑动什么时候停止滑动
    switch (state){
        case 1:
            isScrolling=true;
            isBackScolling=false;                                        //isBackScolling是表示手指离开
            break;
        case 2:
            isBackScolling=true;
            isScrolling=false;
            break;
        default:
            isScrolling=false;
            isBackScolling=false;
            break;

    }

}

@Override
public void onClick(View v) {
    switch (v.getId()){
        case R.id.bt1:
            viewPager.setCurrentItem(0);
            break;
        case R.id.bt2:
            viewPager.setCurrentItem(1);
         break;
        case R.id.bt3:
            viewPager.setCurrentItem(2);
        break;
        case R.id.bt4:
            viewPager.setCurrentItem(3);
            break;
        default:
            break;
    }

}

}

Mainactivity的代码

爆了空指针异常
MainActivity.java:93
LayoutParams lp = tab.getLayoutParams();出错
通过日志可知
Attempt to invoke virtual method 'android.view.ViewGroup$LayoutParams android.widget.ImageView.getLayoutParams()' on a null objectreference
大意是：getLayoutParams()返回的是一个抽象对象ViewGroup的属性LayoutParams 必须返回ViewGroup的子类的LayoutParams
LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) tab.getLayoutParams();

tab= (ImageView) findViewById(R.id.tab);这行代码提前，initLineImage();
这个方法里面你都对tab操作了，那个时候tab还没有初始化

像这种nullponit的话是很好找的，从报错地方按照程序执行的相反顺序找他之前的操作就行了