Layout样式：

layout 代码：

<?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">
    <EditText
        android:id="@+id/edt_intro"
        android:layout_width="350dp"
        android:paddingRight="10dp"
        android:layout_height="wrap_content"
        android:hint="输入.."
        android:paddingBottom="30dp"
        android:textSize="18dp"
        android:gravity="top"
        android:paddingTop="10dp"
        android:paddingLeft="10dp"
        android:textColor="#000000"
        android:textCursorDrawable="@null"
        android:maxLength="400"
        android:minHeight="100dp"
        android:background="@null"
        android:textColorHint="#707070"/>
    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content">
        <TextView
            android:id="@+id/count"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="0/500"
            android:textSize="15dp"
            android:textColor="#707070"
            android:layout_marginTop="15dp"
            android:layout_alignParentRight="true"
            android:layout_marginRight="30dp"/>
    </RelativeLayout>




</LinearLayout>

MainActivity的逻辑实现：

package com.example.counttestapplication;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
    private EditText edt_intro;
    private TextView count;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        edt_intro = findViewById(R.id.edt_intro);
        count = findViewById(R.id.count);
        initEvent();

    }

    private void initEvent() {
        edt_intro.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {

            }

            @Override
            public void afterTextChanged(Editable s) {
                count.setText(String.valueOf(s.length()+"/500"));
                if(s.length()>=500){
                    Toast.makeText(MainActivity.this, "字数超过", Toast.LENGTH_SHORT).show();
                }


            }
        });
    }


}