最简单的登录界面

<?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"
    android:orientation="vertical"
    tools:context=".MainActivity"
    android:background="@drawable/这边自己加图片，图片放左侧drawable"
    android:alpha="0.9"
    >

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"

        >

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="254dp"
            android:orientation="vertical"
            android:gravity="center"
            >

            <EditText
                android:id="@+id/name"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:ems="10"
                android:hint="用户名"
                android:inputType="textPersonName" />

            <EditText
                android:id="@+id/pass"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:ems="10"
                android:hint="密码"
                android:inputType="textPersonName" />

            <LinearLayout
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:orientation="horizontal"
                android:gravity="center"
                >

                <Button
                    android:id="@+id/button"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:hint="登录"

                    />

                <Button
                    android:id="@+id/button2"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="忘记密码" />
            </LinearLayout>

        </LinearLayout>

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical"
            >
        </LinearLayout>
    </LinearLayout>

</androidx.constraintlayout.widget.ConstraintLayout>

//登录验证代码
    @Override
    public void  onClick(View v) {
    
    
        //    通过AlertDialog.Builder这个类来实例化我们的一个AlertDialog的对象
        AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
        //    设置Title的图标
        builder.setIcon(R.drawable.这边自己加图片，图片放左侧drawable);
        //    设置Title的内容


        String mname = "账号";
        String mpass = "密码";
        String user = name.getText().toString().trim();
        String pwd = pass.getText().toString().trim();
        if (user.equals(mname) && pwd.equals(mpass)) {
    
    
            Toast.makeText(this, "登录成功", Toast.LENGTH_SHORT).show();
            builder.setTitle("登录成功");
            builder.show(); //    显示出该对话框
        } else {
    
    
            Toast.makeText(this, "您的输入有误，请重试", Toast.LENGTH_SHORT).show();
            builder.setTitle("您的输入有误，请重试");
            builder.show(); //    显示出该对话框
        }
    }