StringJoiner

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要高效拼接字符串，应该使用StringBuilder。

很多时候，我们拼接的字符串像这样：

// 输出: Hello Bob, Alice, Grace!
public class Main {
    public static void main(String[] args) {
        String[] names = {"Bob", "Alice", "Grace"};
        var sb = new StringBuilder();
        sb.append("Hello ");
        for (String name : names) {
            sb.append(name).append(", ");
        }
        // 注意去掉最后的", ":
        sb.delete(sb.length() - 2, sb.length());
        sb.append("!");
        System.out.println(sb.toString());
    }
}

类似用分隔符拼接数组的需求很常见，所以Java标准库还提供了一个StringJoiner来干这个事：

import java.util.StringJoiner;
public class Main {
    public static void main(String[] args) {
        String[] names = {"Bob", "Alice", "Grace"};
        var sj = new StringJoiner(", ");
        for (String name : names) {
            sj.add(name);
        }
        System.out.println(sj.toString());
    }
}

慢着！用StringJoiner的结果少了前面的"Hello "和结尾的"!"！遇到这种情况，需要给StringJoiner指定“开头”和“结尾”：

import java.util.StringJoiner;
public class Main {
    public static void main(String[] args) {
        String[] names = {"Bob", "Alice", "Grace"};
        var sj = new StringJoiner(", ", "Hello ", "!");
        for (String name : names) {
            sj.add(name);
        }
        System.out.println(sj.toString());
    }
}

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：

String[] names = {"Bob", "Alice", "Grace"};
var s = String.join(", ", names);

练习

请使用StringJoiner构造一个SELECT语句：

import java.util.StringJoiner;

public class Main {
    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) {
        // TODO:
        return "";
    }
}