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app/build.gradle 加上

implementation 'androidx.constraintlayout:constraintlayout:1.1.2'

效果图 app/src/main/res/layout/activity_main.xml

app/src/main/java/com/mycompany/sr/MainActivity.java

package com.mycompany.sr;

import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.design.widget.BottomNavigationView;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentTransaction;
import android.support.v7.app.AppCompatActivity;
import android.view.MenuItem;

public class MainActivity extends AppCompatActivity {

    private BottomNavigationView bottomNavigationView;
    private Fragment fragment1;
    private Fragment fragment2;
    private Fragment fragment3;
    private Fragment[] fragments;
    private int lastfragment;//用于记录上个选择的Fragment

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // listView = (ListView) findViewById(R.id.mainList);
        BottomNavigationView navigation = findViewById(R.id.navigation);
		initFragment();
    }
	
    private void initFragment() {
        fragment1 = new Sub1Activity();
        fragment2 = new Sub2Activity();
        fragment3 = new Sub3Activity();
        fragments = new Fragment[]{fragment1, fragment2, fragment3};
        lastfragment = 0;
        getSupportFragmentManager().beginTransaction().replace(R.id.mainView, fragment1).show(fragment1).commit();
        bottomNavigationView = (BottomNavigationView) findViewById(R.id.navigation);

        bottomNavigationView.setOnNavigationItemSelectedListener(changeFragment);
    }

    private BottomNavigationView.OnNavigationItemSelectedListener changeFragment = new BottomNavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {

            switch (item.getItemId()) {
                case R.id.navigation_home: {
						if (lastfragment != 0) {
							switchFragment(lastfragment, 0);
							lastfragment = 0;
						}
						return true;
					}
                case R.id.navigation_dashboard: {
						if (lastfragment != 1) {
							switchFragment(lastfragment, 1);
							lastfragment = 1;

						}

						return true;
					}
                case R.id.navigation_notifications: {
						if (lastfragment != 2) {
							switchFragment(lastfragment, 2);
							lastfragment = 2;
						}
						return true;
					}
            }
            return false;
        }
    };

    //切换Fragment
    private void switchFragment(int lastfragment, int index) {
        FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
        transaction.hide(fragments[lastfragment]);//隐藏上个Fragment
        if (fragments[index].isAdded() == false) {
            transaction.add(R.id.mainView, fragments[index]);
        }
        transaction.show(fragments[index]).commitAllowingStateLoss();
    }
	
}

下面创建三个 Activity和相对应的xml界面 app/src/main/java/com/mycompany/sr/Sub1Activity.java

package com.mycompany.sr;

import android.support.annotation.Nullable;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;

public class Sub1Activity extends Fragment {
    private TextView textView;
    private Button button;
    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        //不同的Activity对应不同的布局
        View view = inflater.inflate(R.layout.activity_sub1, container, false);
        return view;
    }

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        /**
         * TODO 实现底部菜单对应布局控件事件
         * */

    }

}

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