// 定义与文件名Main.java同名的公共类（Java强制要求）
public class Main {
    // Java程序固定入口方法，所有可执行代码必须放在这里
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();
        sb.append("hi");
 
        // String str = sb; // 报错，类型不匹配，sb是StringBuilder，不是String
        // 调用toString()将StringBuilder转为String类型
        String str = sb.toString();
    }
}
 

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