Android Studio报‘class’ or ‘interface’ expected错误，代码在线编译正常

问题原因分析 #

你碰到的这个错误核心是代码结构违反了Java类的语法规则：你的onCreate方法被错误地放在了Activity2类的外部——看代码就能发现，在rand()方法结束后，你用一个}把Activity2类给闭合了，之后才写@Override和onCreate方法。Java里方法必须定义在类内部，所以编译器才会提示需要class或interface。

另外补充个小知识点：Android应用的启动入口不是main方法，而是Activity的生命周期方法（比如onCreate），你写的main方法在Android环境下根本用不上，可以直接移除；如果想留着做本地测试，也要确保它在类的内部。

修正后的代码 #

package com.tech.littlest.myapplication;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import java.util.Random;

public class Activity2 extends AppCompatActivity {
    // 注意：proposition_user数组有13个元素，这里常量要改成13，避免数组越界
    final static int NO_DEDICATED_ARRAYS = 13; 
    final static int NO_OUTPUT_SENTENCES = 1;
    final static String SPACER = " ";
    final static String PERIOD = ".";
    static Random r = new Random();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_2);
        // 在Activity启动时调用生成句子的逻辑
        generateRandomSentences();
    }

    // 把原main方法里的逻辑抽成独立方法，便于在Android生命周期中调用
    private void generateRandomSentences() {
        String proposition_user[] = { "You should build an invention that", "You should build a robot that", "You should create a whatchamacallit that", "There should be a device that", "Create a futuristic thing that", "Make a thing that", "Make something cool that", "I don't recall there being a thing that", "Create a sick thing that", "Devise a plan to create an object that", "I want a thing that", "If I just had a thingamajig that", "Create a thingimajig that"};
        String plural_noun[] = { "children", "chickens", "the ocean", "cars", "apples", "houses", "water", "buildings", "people", "your friends", "farms", "the human race" };
        String verb[] = { "directs you to", "drives on", "manages", "develops", "makes","eats", "helps", "relocates", "fixes", "feeds", "runs on", "washes" };
        
        String sentence;
        for (int i = 0; i < NO_OUTPUT_SENTENCES; i++){
            sentence = proposition_user[rand()];
            char c = sentence.charAt(0);
            sentence = sentence.replace(c, Character.toUpperCase(c));
            sentence += SPACER;
            sentence += verb[rand()];
            sentence += SPACER + plural_noun[rand()];
            sentence += PERIOD;
            // Android里用Log打印日志，替代System.out.println
            Log.d("RandomSentence", sentence);
        }
    }

    static int rand(){
        int ri = r.nextInt() % NO_DEDICATED_ARRAYS;
        if (ri < 0) ri += NO_DEDICATED_ARRAYS;
        return ri;
    }
}

额外优化提示 #

1. 我帮你修正了proposition_user数组里的拼写错误（and invention改成an invention、an thingamajig改成a thingamajig），同时把NO_DEDICATED_ARRAYS改成13，避免数组越界异常。
2. Android环境下不要用System.out.println输出内容，推荐用Log类打印日志，或者把生成的句子显示在界面的TextView组件中。

内容的提问来源于stack exchange，提问作者Aaron Kauffman