九、字符串补充

1、StringBuilder

String s = "";
for (int i=0;i<100;i++){
    s = s + "," + i;
}
/*每次拼接字符串都会丢掉旧的字符串，造成浪费内存；
为了高效拼接字符串，java标准库提供了StringBuilder,
它是一个可变对象，可以与存放缓冲区，增加字符时，不会创建新的临时对象
*/
public class Playing{
    public static void main(String[] args) {
        StringBuilder p = new StringBuilder(1024);
        for (int i= 0;i<100;i++){
            p.append(',');
            p.append(i);
        }
        String s = p.toString();
        System.out.println(s);
    }
}

2、StringJoiner

// 用分割符拼接数组的需求
import java.util.StringJoiner;
public class Playing{
    public static void main(String[] args) {
        String[] names = {"Bob","Alice","Grace"};
        var p = new StringJoiner(", ","Hello ", "!");
        // StringJoiner("分隔符","前缀", "后缀")
        for (String name : names) {
            p.add(name);
        }
        System.out.println(p.toString());
    }
}
// 实例
import java.util.StringJoiner;
public class Playing {
    public static void main(String[] args) {
        String[] fields = {"name", "position", "salary"};
        String table = "employee";
        String select = buildSelectSql(table, fields);
        System.out.println(select);
        System.out.println("SELECT name, position, salary FROM employee".equals(select) ? "测试成功" : "测试失败");
    }
    static String buildSelectSql(String table, String[] fields) {
        var sql = new StringJoiner(", ", "SELECT ", " FROM " + table);
        for (String field : fields) {
            sql.add(field);
        }
        return sql.toString();
    }
}

3、String.join()

/*String还提供了一个静态方法join()，这个方法在内部使用了StringJoiner来拼接字符串，在不需要指定“开头”和“结尾”的时候，用String.join()更方便*/
public class Playing{
    public static void main(String[] args) {
        String[] names = {"Bob","Alice","Grace"};
        var s = String.join(", ",names);
        System.out.println(s);
    }
}