BottomNavigationView为空

Question

我在实现BottomNavigationView时有一个运行时异常，

由: com.google.android.material.bottomnavigation.BottomNavigationView.setOnNavigationItemSelectedListener(com.google.android.material.bottomnavigation.BottomNavigationView$OnNavigationItemSelectedListener)‘引起的

：尝试在空对象引用上调用虚拟方法的void

这是我的代码：

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;

import android.os.Bundle;
import android.util.Log;
import android.view.MenuItem;

import com.google.android.material.bottomnavigation.BottomNavigationView;

public class MainActivity extends AppCompatActivity {
    private BottomNavigationView bottom_nav;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        bottom_nav=  findViewById(R.id.bottom_nav);
        getSupportFragmentManager().beginTransaction().add(R.id.fragTutucu, new Frag1());

        bottom_nav.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
                Fragment fragment = null;
                switch (menuItem.getItemId()){
                    case R.id.manuel:
                        fragment= new Frag1();
                        return true;
                    case R.id.photo:
                        fragment= new Frag2();
                        return true;
                    case R.id.training:
                        fragment= new Frag3();
                        return true;

                }
                getSupportFragmentManager().beginTransaction().replace(R.id.fragTutucu,fragment);
                return true;

            }
        });


        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

Answer 1

您正在尝试在设置活动视图之前找到BottomNavigationView。您应该首先设置content视图。

按照以下方式重新组织代码

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

    // everything other
}

Answer 2

嗨，您没有对XML文件提供任何引用，包括onCreate方法开头的这些行

super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);