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主界面一般来说都是底部几个按钮，点击按钮可切换不同页面。一般来说我们是用fragment来实现的。给按钮设置监听事件，监听到点击按钮时候 展示对应fragmnt，而实现fragment方式有两种，一种是静态的 建一个继承fragment类重写onCreateView方法 写一个xml文件关联上（这里xml里要写上对应的是哪个fragment文件要不然找不到）然后在主界面xml文件里直接fragment控件引入进来就可以了。但是一般我们不用这种写法，这种简单的fragment还可以 复杂的就不行了。我们一般用第二种 动态的。用法同样需要写一个继承fragment的类及对应xml文件（不需要写对应哪个fragment），不同的是主界面activity的处理，需要这么几步

【1】Activity中获得FragmentManager对象，通过getSupportFragmentManager()

FragmentManager fm = getSupportFragmentManager();

【2】获得FragmentTransaction对象，通过fm.beginTransaction()

FragmentTransaction ft = fm.beginTransaction();

【3】调用add()方法或者repalce()方法加载Fragment;

Fragment fl = new mFragment1(); ft.add(R.id.fragment,fl);

【4】最后调用commit()方法提交事务

ft.commit();

本来这个也挺简单的，因为急着赶项目  于是乎我便在网上找了个示例直接贴过来了（之前也用过fragment 想着没啥坑） 狗血的事情开始了直接报错

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.recanapp/com.example.recanapp.sjjk.SjjkActivity}: java.lang.IllegalArgumentException: No view found for id 0x7f0800f6 (com.example.recanapp:id/sjjk_fdj_fragment) for fragment fdjFragment{93a9f95 (0865f1ea-90ee-43df-aeb4-01e0d5ae908f) id=0x7f0800f6}

at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2451)

at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2511)

at android.app.ActivityThread.access$900(ActivityThread.java:165)

at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1375)

at android.os.Handler.dispatchMessage(Handler.java:102)

at android.os.Looper.loop(Looper.java:150)

at android.app.ActivityThread.main(ActivityThread.java:5621)

at java.lang.reflect.Method.invoke(Native Method)

at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:794)

at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:684)

Caused by: java.lang.IllegalArgumentException: No view found for id 0x7f0800f6 (com.example.recanapp:id/sjjk_fdj_fragment) for fragment fdjFragment{93a9f95 (0865f1ea-90ee-43df-aeb4-01e0d5ae908f) id=0x7f0800f6}

at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:875)

at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1238)

at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:434)

找不到对应fragment的view，我便开始排查fragment的xml文件 如图所示

就一个文本框还没开始往里写东西呢  这也没错呀。难道要find一下这个文件结果又报了fragment空对象问题  刚new出来对象这么是空呢  难道是我导入包不对，可是如果是这样原因 应该报对应错呀。于是认认真真排查代码 发现是transaction.add(R.id.one_fragment, onefragment);时候错了。这里应该是装这个fragment容器id而不是fragmen文件的id。我们在activity里面xml文件里会用一个帧布局来装fragment 如图

这个transaction对象add的应该是帧布局也就是装fragment父布局id 而不是fragment本身id

找到问题当然就好解决了

贴上修改后activity核心代码

private TextView tab01, tab02, tab03, tab04;

tab01 = findViewById(R.id.fdj);
tab02 = findViewById(R.id.yyxt);
tab03 = findViewById(R.id.dqxt);
tab04 = findViewById(R.id.qt);

/*为四个按钮绑定点击监听*/
tab01.setOnClickListener(this);
tab02.setOnClickListener(this);
tab03.setOnClickListener(this);
tab04.setOnClickListener(this);

/*创建四个Fragment对象*/
private Fragment fdj_Fragment;
private Fragment yyxt_Fragment;
private Fragment dqxt_Fragment;
private Fragment qt_Fragment;

/*将四个Fragment对象创建为对应的Fragment*/
fdj_Fragment = new fdjFragment();
yyxt_Fragment = new yyxtFragment();
dqxt_Fragment = new dqxtFragment();
qt_Fragment = new qtFragment();

/*创建一个FragmentManager和FragmentTransaction*/
private FragmentManager manager;
private FragmentTransaction transaction;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    //remove title bar  即隐藏标题栏
    //requestWindowFeature(Window.FEATURE_NO_TITLE);
    // 隐藏ActionBar
    //getSupportActionBar().hide();
    //remove notification bar  即全屏
    //getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);
    setContentView(R.layout.sjjk_activity);
    init();
    initFragment();
    selectTab(1);//默认选中第一个Tab
}

@Override
public void onClick(View view) {
    //先将四个Button置为黑色
    buttonColor();
    switch (view.getId()) {
 case R.id.fdj:
        selectTab(1);
        break;
    case R.id.yyxt:
        selectTab(2);
        break;
    case R.id.dqxt:
        selectTab(3);
        break;
    case R.id.qt:
        selectTab(4);
        break;
    default:
        break;
}

}

//进行选中Tab的处理
private void selectTab(int i) {
    //获取FragmentManager对象
    FragmentManager manager = getSupportFragmentManager();
    //获取FragmentTransaction对象
    FragmentTransaction transaction = manager.beginTransaction();
    //先隐藏所有的Fragment
    hideFragments(transaction);
    switch (i) {
        //当选中点击的是第一页的Tab时
        case 1:
            //设置第一页的Button为铜色
            tab01.setBackgroundResource(R.color.Copper);
            //如果第一页对应的Fragment没有实例化，则进行实例化，并显示出来
            if (fdj_Fragment == null) {
                fdj_Fragment = new dqxtFragment();
            }
            if (fdj_Fragment.isAdded() == false) {
                transaction.add(R.id.sjjk_fragment, fdj_Fragment);
            }
            transaction.show(fdj_Fragment);
            break;
        case 2:
            tab02.setBackgroundResource(R.color.Copper);
            if (yyxt_Fragment == null) {
                yyxt_Fragment = new yyxtFragment();
            }
            if (yyxt_Fragment.isAdded() == false){
                transaction.add(R.id.sjjk_fragment, yyxt_Fragment);
            }
            transaction.show(yyxt_Fragment);
            break;
        case 3:
            tab03.setBackgroundResource(R.color.Copper);
            if (dqxt_Fragment == null) {
                dqxt_Fragment = new dqxtFragment();
            }
            if (dqxt_Fragment.isAdded() == false){
                transaction.add(R.id.sjjk_fragment, dqxt_Fragment);
            }
            transaction.show(dqxt_Fragment);
            break;
        case 4:
            tab04.setBackgroundResource(R.color.Copper);
            if (qt_Fragment == null) {
                qt_Fragment = new qtFragment();
            }
            if(qt_Fragment.isAdded() == false) {
                transaction.add(R.id.sjjk_fragment, qt_Fragment);
            }
            transaction.show(qt_Fragment);
            break;
    }
    //不要忘记提交事务
    transaction.commit();
}

/*隐藏Fragment*/
private void hideFragments(FragmentTransaction transaction) {
    if (fdj_Fragment != null) {
        transaction.hide(fdj_Fragment);
    }
    if (yyxt_Fragment != null) {
        transaction.hide(yyxt_Fragment);
    }
    if (dqxt_Fragment != null) {
        transaction.hide(dqxt_Fragment);
    }
    if (qt_Fragment != null) {
        transaction.hide(qt_Fragment);
    }
}

private void buttonColor() {
    tab01.setBackgroundResource(R.color.black);
    tab02.setBackgroundResource(R.color.black);
    tab03.setBackgroundResource(R.color.black);
    tab04.setBackgroundResource(R.color.black);
}