Android基本控件使用

学习目标：掌握Button、Log、Toast、EditText的几种常见使用方式

Button

方式一：直接在布局文件当中添加

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/button"
        android:layout_height="wrap_content"
        android:layout_width="wrap_content"
        android:onClick="test"
        android:text="Button"
        tools:ignore="MissingConstraints" />

</androidx.constraintlayout.widget.ConstraintLayout>

package com.example.myapplication;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.TextView;

import com.example.myapplication.databinding.ActivityMainBinding;

public class MainActivity extends AppCompatActivity {

    // Used to load the 'myapplication' library on application startup.
    static {
        System.loadLibrary("myapplication");
    }

    private ActivityMainBinding binding;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        binding = ActivityMainBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());

    }

    /**
     * A native method that is implemented by the 'myapplication' native library,
     * which is packaged with this application.
     */
    public native String stringFromJNI();

    public void test(View view) {
        Log.d("hyq", "按钮被点击了");
    }
}

方式二：使用匿名类

public class MainActivity extends AppCompatActivity {

    // Used to load the 'myapplication' library on application startup.
    static {
        System.loadLibrary("myapplication");
    }

    private ActivityMainBinding binding;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        binding = ActivityMainBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());
        Button button = findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Log.d("hyq", "按钮被点击了");
            }
        });

    }

    /**
     * A native method that is implemented by the 'myapplication' native library,
     * which is packaged with this application.
     */
    public native String stringFromJNI();
}

方法三：使用自定义类

package com.example.myapplication;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import com.example.myapplication.databinding.ActivityMainBinding;

public class MainActivity extends AppCompatActivity {

    // Used to load the 'myapplication' library on application startup.
    static {
        System.loadLibrary("myapplication");
    }

    private ActivityMainBinding binding;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        binding = ActivityMainBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());
        Button button = findViewById(R.id.button);
        button.setOnClickListener(new MyClass());
    }

    /**
     * A native method that is implemented by the 'myapplication' native library,
     * which is packaged with this application.
     */
    public native String stringFromJNI();
}
class MyClass implements View.OnClickListener{

    @Override
    public void onClick(View view) {
        Log.d("hyq", "按钮被点击了");
    }
}

方式四：MainActivity实现接口

package com.example.myapplication;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import com.example.myapplication.databinding.ActivityMainBinding;

public class MainActivity extends AppCompatActivity implements View.OnClickListener{

    // Used to load the 'myapplication' library on application startup.
    static {
        System.loadLibrary("myapplication");
    }

    private ActivityMainBinding binding;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        binding = ActivityMainBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());
        Button button = findViewById(R.id.button);
        //几种绑定方式
        button.setOnClickListener(new MainActivity());
        button.setOnClickListener(MainActivity.this);
        button.setOnClickListener(this);

    }

    /**
     * A native method that is implemented by the 'myapplication' native library,
     * which is packaged with this application.
     */
    public native String stringFromJNI();

    @Override
    public void onClick(View view) {
        Log.d("hyq", "按钮被点击了");
    }
}

假如界面上有很多个button便可以使用这种定义方式来减少重复代码

Button button1 = findViewById(R.id.button1);
Button button2 = findViewById(R.id.button2);
Button button3 = findViewById(R.id.button3);
Button button4 = findViewById(R.id.button4);
Button button5 = findViewById(R.id.button5);
public void onClick(View view) {
    switch(view.getId()){
        case R.id.button1:
            Log.d("hyq", "按钮被点击了");
            break;
    }
    Log.d("hyq", "其他情况");
}

Toast

Toast.makeText(MainActivity.this, "hyq", Toast.LENGHT.LENGHT_LONG);

EditText

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/button"
        android:layout_height="wrap_content"
        android:layout_width="wrap_content"
        android:onClick="test"
        android:text="Button"
        tools:ignore="MissingConstraints" />
    <EditText
        android:id="@+id/edittext"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inputType="textPersonName"
        android:ems="10"
        />
</LinearLayout>

package com.example.myapplication;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import com.example.myapplication.databinding.ActivityMainBinding;

public class MainActivity extends AppCompatActivity implements View.OnClickListener{

    EditText editText;

    // Used to load the 'myapplication' library on application startup.
    static {
        System.loadLibrary("myapplication");
    }

    private ActivityMainBinding binding;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        binding = ActivityMainBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());
        Button button = findViewById(R.id.button);
        button.setOnClickListener(this);
        editText = findViewById(R.id.edittext);

    }

    /**
     * A native method that is implemented by the 'myapplication' native library,
     * which is packaged with this application.
     */
    public native String stringFromJNI();

    @Override
    public void onClick(View view) {
        String username = editText.getText().toString().trim();
        Log.d("hyq", "按钮被点击了" + username);
    }
}

1. username!= null

此条件用于检查 username 这个字符串对象是否为 null。在 Java 中，null 代表对象引用不指向任何实际的对象。要是 username 为 null，那么调用 username.length() 方法时就会抛出 NullPointerException 异常。所以，先检查 username 是否为 null 是为了避免出现这种异常。

2. username.length() != 0

该条件用于检查 username 字符串的长度是否为 0。即使 username 不是 null，它也可能是一个空字符串（例如 ""）。当字符串为空时，其长度为 0。这个条件可以确保输入的字符串不为空。