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);
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 "";
StringJoiner练习
用指定分隔符拼接字符串数组时，使用StringJoiner或者String.join()更方便；
用StringJoiner拼接字符串时，还可以额外附加一个“开头”和“结尾”。