Java 重复字符串的简单方法

我正在寻找一个简单的commons方法或操作符，它允许我将一些字符串重复n次。我知道我可以用for循环来写这个，但是我希望在必要的时候避免for循环，并且应该有一个简单的直接方法

String str = "abc";
String repeated = str.repeat(3);

repeated.equals("abcabcabc");

与以下内容相关：

已编辑

我尽量避免在不完全必要时使用for循环，因为：

---

它们增加了代码行数，即使它们被隐藏在另一个函数中

阅读我的代码的人必须弄清楚我在for循环中做了什么。即使它被注释并且有有有意义的变量名，他们仍然必须确保它没有做任何“聪明”的事情

程序员喜欢把聪明的东西放进循环中，即使我写它是为了“只做它想做的事”，但这并不妨碍有人来添加一些额外的聪明的“修复”

它们往往很容易出错。对于涉及索引的循环，往往会生成一个错误

For循环经常重用相同的变量，这增加了很难找到范围错误的机会

对于循环，增加臭虫猎人必须寻找的地方的数量

用法：

String str = "abc";
String repeated = StringUtils.repeat(str, 3);

repeated.equals("abcabcabc");

---

如果您担心性能，只需在循环中使用StringBuilder，并在循环退出时执行.toString（）。见鬼，编写自己的Util类并重用它。最多5行代码。

这包含的字符比您的问题少

public static String repeat(String s, int n) {
    if(s == null) {
        return null;
    }
    final StringBuilder sb = new StringBuilder(s.length() * n);
    for(int i = 0; i < n; i++) {
        sb.append(s);
    }
    return sb.toString();
}

公共静态字符串重复（字符串s，int n）{
如果（s==null）{
返回null；
}
最终StringBuilder sb=新StringBuilder（s.长度（）*n）；
对于（int i=0；i

仅使用JRE类（）和尽量减少可以编写以下内容的临时对象的数量：

public static String repeat(String toRepeat, int times) {
    if (toRepeat == null) {
        toRepeat = "";
    }

    if (times < 0) {
        times = 0;
    }

    final int length = toRepeat.length();
    final int total = length * times;
    final char[] src = toRepeat.toCharArray();
    char[] dst = new char[total];

    for (int i = 0; i < total; i += length) {
        System.arraycopy(src, 0, dst, i, length);
    }

    return String.copyValueOf(dst);
}

---

但是我仍然喜欢第一个版本。

尽管您希望不使用循环，但我认为您应该使用循环

String repeatString(String s, int repetitions)
{
    if(repetitions < 0) throw SomeException();

    else if(s == null) return null;

    StringBuilder stringBuilder = new StringBuilder(s.length() * repetitions);

    for(int i = 0; i < repetitions; i++)
        stringBuilder.append(s);

    return stringBuilder.toString();
}

字符串重复字符串（字符串s，整数重复）
{
如果（重复次数<0）抛出SomeException（）；
如果（s==null），则返回null；
StringBuilder StringBuilder=新StringBuilder（s.length（）*重复）；
for（int i=0；i<重复；i++）
stringBuilder.append；
返回stringBuilder.toString（）；
}

您不使用for循环的原因不是很好。针对您的批评：

无论您使用什么解决方案，几乎肯定都会比这更长。使用预先构建的功能只会将其塞进更多的盖子中

阅读你的代码的人必须弄清楚你在非for循环中做了什么。考虑到for循环是实现这一点的惯用方法，如果使用for循环来实现这一点，将更容易理解

是的，有人可能会添加一些聪明的东西，但是通过避免for循环，你正在做一些聪明的事情。这就像是故意朝自己的脚开枪，以避免意外地朝自己的脚开枪

一个接一个的错误也很容易被一次测试抓住。考虑到您应该测试代码，逐个错误应该很容易修复和捕获。值得注意的是：上面的代码不包含off-by-one错误。For循环同样容易正确

所以不要重用变量。这不是for循环的错

同样，你使用的任何解决方案也是如此。正如我之前提到的；bug搜寻者可能希望您使用for循环来完成这项工作，因此如果您使用for循环，他们将更容易找到它

---

所以你想避免循环

给你：

public static String repeat(String s, int times) {
    if (times <= 0) return "";
    else return s + repeat(s, times-1);
}

编辑：让我们稍微优化一下：-D

public static String repeat(String s, int times) {
   if (times <= 0) return "";
   else if (times % 2 == 0) return repeat(s+s, times/2);
   else return s + repeat(s+s, times/2);
}

公共静态字符串重复（字符串s，整数倍）{
如果（times基于，则这是一个使用StringBuilder的递归版本：

public static void repeat(StringBuilder stringBuilder, String s, int times) {
    if (times > 0) {
        repeat(stringBuilder.append(s), s, times - 1);
    }
}

public static String repeat(String s, int times) {
    StringBuilder stringBuilder = new StringBuilder(s.length() * times);
    repeat(stringBuilder, s, times);
    return stringBuilder.toString();
}

 /**
   * Repeat a String as many times you need.
   *
   * @param i - Number of Repeating the String.
   * @param s - The String wich you want repeated.
   * @return The string n - times.
   */
  public static String repeate(int i, String s) {
    StringBuilder sb = new StringBuilder();
    for (int j = 0; j < i; j++)
      sb.append(s);
    return sb.toString();
  }

以下是一种仅使用标准字符串函数而不使用显式循环的方法：

// create a string made up of  n  copies of  s
repeated = String.format(String.format("%%%ds", n), " ").replace(" ",s);

使用的方法很简单，只需键入：

@Test
public void repeatString() {
    String string = "abc";
    assertThat($(string).repeat(3).toString(), is("abcabcabc"));
}

PS:repeat也适用于数组、列表、集合等
这里是最短的版本（Java 1.5+必需）：

其中，
n
是要重复字符串的次数，
s
是要重复的字符串

无需导入或库。
如果您使用的是Java这里是最新的Stringutils.Java

publicstaticstringrepeat（stringstr，int repeat）{
//针对2.0（JDK1.4）进行性能调整
如果（str==null）{
返回null；
}
如果（重复如果你像我一样，想使用Google Guava而不是Apache Commons，你可以在Guava Strings类中使用这个方法

Strings.repeat("-", 60);

我想要一个函数来为JDBC创建一个逗号分隔的问号列表，并找到了这篇文章。因此，我决定采用两个变体，看看哪一个性能更好。经过100万次迭代后，garden variety StringBuilder花了2秒（fun1），而神秘的本应是更优化的版本（fun2）花了30秒。再次神秘又有什么意义

private static String fun1(int size) {
    StringBuilder sb = new StringBuilder(size * 2);
    for (int i = 0; i < size; i++) {
        sb.append(",?");
    }
    return sb.substring(1);
}

private static String fun2(int size) {
    return new String(new char[size]).replaceAll("\0", ",?").substring(1);
}

private静态字符串fun1（int-size）{
StringBuilder sb=新的StringBuilder（尺寸*2）；
对于（int i=0；i
我真的很喜欢这个问题。有很多知识和风格。所以我不能不展示一下我的摇滚乐；）

喜欢吗？
公共静态字符串重复（字符串str，int次）{
public static String repeat(String str, int times) {
    int length = str.length();
    int size = length * times;
    char[] c = new char[size];
    for (int i = 0; i < size; i++) {
        c[i] = str.charAt(i % length);
    }
    return new String(c);
}

int length=str.length（）；
整数大小=长度*倍；
字符[]c=新字符[大小]；
对于（int i=0；i
简单循环

public static String repeat(String string, int times) {
    StringBuilder out = new StringBuilder();
    while (times-- > 0) {
        out.append(string);
    }
    return out.toString();
}

试试这个：

public static char[] myABCs = {'a', 'b', 'c'};
public static int numInput;
static Scanner in = new Scanner(System.in);

public static void main(String[] args) {
    System.out.print("Enter Number of Times to repeat: ");
    numInput = in.nextInt();
    repeatArray(numInput);
}

public static int repeatArray(int y) {
    for (int a = 0; a < y; a++) {
        for (int b = 0; b < myABCs.length; b++) {
            System.out.print(myABCs[b]);                
        }
        System.out.print(" ");
    }
    return y;
}

publicstaticchar[]myABCs={'a'，'b'，'c'}；
努明普公共静态int
// create a string made up of n copies of string s
String.join("", Collections.nCopies(n, s));

"abc".repeat(12);

StringUtils.repeat("abc", 12);

Strings.repeat("abc", 12);

    public static String repeat(String str, int repeat) {
    // Performance tuned for 2.0 (JDK1.4)

    if (str == null) {
        return null;
    }
    if (repeat <= 0) {
        return EMPTY;
    }
    int inputLength = str.length();
    if (repeat == 1 || inputLength == 0) {
        return str;
    }
    if (inputLength == 1 && repeat <= PAD_LIMIT) {
        return repeat(str.charAt(0), repeat);
    }

    int outputLength = inputLength * repeat;
    switch (inputLength) {
        case 1 :
            return repeat(str.charAt(0), repeat);
        case 2 :
            char ch0 = str.charAt(0);
            char ch1 = str.charAt(1);
            char[] output2 = new char[outputLength];
            for (int i = repeat * 2 - 2; i >= 0; i--, i--) {
                output2[i] = ch0;
                output2[i + 1] = ch1;
            }
            return new String(output2);
        default :
            StringBuilder buf = new StringBuilder(outputLength);
            for (int i = 0; i < repeat; i++) {
                buf.append(str);
            }
            return buf.toString();
    }
    }

    public static String repeat(String str, int num) {
    int len = num * str.length();
    StringBuilder sb = new StringBuilder(len);
    for (int i = 0; i < times; i++) {
        sb.append(str);
    }
    return sb.toString();
    }

Strings.repeat("-", 60);

private static String fun1(int size) {
    StringBuilder sb = new StringBuilder(size * 2);
    for (int i = 0; i < size; i++) {
        sb.append(",?");
    }
    return sb.substring(1);
}

private static String fun2(int size) {
    return new String(new char[size]).replaceAll("\0", ",?").substring(1);
}

{
    String string = repeat("1234567890", 4);
    System.out.println(string);
    System.out.println("=======");
    repeatWithoutCopySample(string, 100000);
    System.out.println(string);// This take time, try it without printing
    System.out.println(string.length());
}

/**
 * The core of the task.
 */
@SuppressWarnings("AssignmentToMethodParameter")
public static char[] repeat(char[] sample, int times) {
    char[] r = new char[sample.length * times];
    while (--times > -1) {
        System.arraycopy(sample, 0, r, times * sample.length, sample.length);
    }
    return r;
}

/**
 * Java classic style.
 */
public static String repeat(String sample, int times) {
    return new String(repeat(sample.toCharArray(), times));
}

/**
 * Java extreme memory style.
 */
@SuppressWarnings("UseSpecificCatch")
public static void repeatWithoutCopySample(String sample, int times) {
    try {
        Field valueStringField = String.class.getDeclaredField("value");
        valueStringField.setAccessible(true);
        valueStringField.set(sample, repeat((char[]) valueStringField.get(sample), times));
    } catch (Exception ex) {
        throw new RuntimeException(ex);
    }
}

public static String repeat(String str, int times) {
    int length = str.length();
    int size = length * times;
    char[] c = new char[size];
    for (int i = 0; i < size; i++) {
        c[i] = str.charAt(i % length);
    }
    return new String(c);
}

public static String repeat(String string, int times) {
    StringBuilder out = new StringBuilder();
    while (times-- > 0) {
        out.append(string);
    }
    return out.toString();
}

public static char[] myABCs = {'a', 'b', 'c'};
public static int numInput;
static Scanner in = new Scanner(System.in);

public static void main(String[] args) {
    System.out.print("Enter Number of Times to repeat: ");
    numInput = in.nextInt();
    repeatArray(numInput);
}

public static int repeatArray(int y) {
    for (int a = 0; a < y; a++) {
        for (int b = 0; b < myABCs.length; b++) {
            System.out.print(myABCs[b]);                
        }
        System.out.print(" ");
    }
    return y;
}

// say hello 100 times
System.out.println(String.join("", Collections.nCopies(100, "hello")));

import static java.util.stream.Collectors.joining;
...
String repeated = Stream.generate(() -> "abc").limit(3).collect(joining()); //"abcabcabc"

public static String repeat(String str, int times) {
   return Stream.generate(() -> str).limit(times).collect(joining());
}

public static final String repeat(String string, long number) {
    return number == 1 ? string : (number % 2 == 0 ? repeat(string + string, number / 2) : string + repeat(string + string, (number - 1) / 2));
}

/**
 * Helper-Class for Repeating Strings and other CharSequence-Implementations
 * @author Maciej Schuttkowski
 */
public class RepeatingCharSequence implements CharSequence {
    final int count;
    CharSequence internalCharSeq = "";
    CharSequence separator = "";
    /**
     * CONSTRUCTOR - RepeatingCharSequence
     * @param input CharSequence to repeat
     * @param count Repeat-Count
     */
    public RepeatingCharSequence(CharSequence input, int count) {
        if(count < 0)
            throw new IllegalArgumentException("Can not repeat String \""+input+"\" less than 0 times! count="+count);
        if(count > 0)
            internalCharSeq = input;
        this.count = count;
    }
    /**
     * CONSTRUCTOR - Strings.RepeatingCharSequence
     * @param input CharSequence to repeat
     * @param count Repeat-Count
     * @param separator Separator-Sequence to use
     */
    public RepeatingCharSequence(CharSequence input, int count, CharSequence separator) {
        this(input, count);
        this.separator = separator;
    }

    @Override
    public CharSequence subSequence(int start, int end) {
        checkBounds(start);
        checkBounds(end);
        int subLen = end - start;
        if (subLen < 0) {
            throw new IndexOutOfBoundsException("Illegal subSequence-Length: "+subLen);
        }
        return (start == 0 && end == length()) ? this
                    : toString().substring(start, subLen);
    }
    @Override
    public int length() {
        //We return the total length of our CharSequences with the separator 1 time less than amount of repeats:
        return count < 1 ? 0
                : ( (internalCharSeq.length()*count) + (separator.length()*(count-1)));
    }
    @Override
    public char charAt(int index) {
        final int internalIndex = internalIndex(index);
        //Delegate to Separator-CharSequence or Input-CharSequence depending on internal index:
        if(internalIndex > internalCharSeq.length()-1) {
            return separator.charAt(internalIndex-internalCharSeq.length());
        }
        return internalCharSeq.charAt(internalIndex);
    }
    @Override
    public String toString() {
        return count < 1 ? ""
                : new StringBuilder(this).toString();
    }

    private void checkBounds(int index) {
        if(index < 0 || index >= length())
            throw new IndexOutOfBoundsException("Index out of Bounds: "+index);
    }
    private int internalIndex(int index) {
        // We need to add 1 Separator-Length to total length before dividing,
        // as we subtracted one Separator-Length in "length()"
        return index % ((length()+separator.length())/count);
    }
}

public static void main(String[] args) {
    //String input = "12345";
    //StringBuffer input = new StringBuffer("12345");
    StringBuilder input = new StringBuilder("123");
    //String separator = "<=>";
    StringBuilder separator = new StringBuilder("<=");//.append('>');
    int repeatCount = 2;

    CharSequence repSeq = new RepeatingCharSequence(input, repeatCount, separator);
    String repStr = repSeq.toString();

    System.out.println("Repeat="+repeatCount+"\tSeparator="+separator+"\tInput="+input+"\tLength="+input.length());
    System.out.println("CharSeq:\tLength="+repSeq.length()+"\tVal="+repSeq);
    System.out.println("String :\tLength="+repStr.length()+"\tVal="+repStr);

    //Here comes the Magic with a StringBuilder as Input, as you can append to the String-Builder
    //and at the same Time your Repeating-Sequence's toString()-Method returns the updated String :)
    input.append("ff");
    System.out.println(repSeq);
    //Same can be done with the Separator:
    separator.append("===").append('>');
    System.out.println(repSeq);
}

Repeat=2    Separator=<=    Input=123   Length=3
CharSeq:    Length=8    Val=123<=123
String :    Length=8    Val=123<=123
123ff<=123ff
123ff<====>123ff

public String repeat(String str, int count) {
    return count > 0 ?  repeat(str, count -1) + str: "";
}

public String repeat(String str, int count){
    if(count <= 0) {return "";}
    return new String(new char[count]).replace("\0", str);
}

public static String repeatString(String what, int howmany) {
    char[] pattern = what.toCharArray();
    char[] res = new char[howmany * pattern.length];
    int length = pattern.length;
    for (int i = 0; i < howmany; i++)
        System.arraycopy(pattern, 0, res, i * length, length);
    return new String(res);
}

public static String repeatStringSB(String what, int howmany) {
    StringBuilder out = new StringBuilder(what.length() * howmany);
    for (int i = 0; i < howmany; i++)
        out.append(what);
    return out.toString();
}

public static void main(String... args) {
    String res;
    long time;

    for (int j = 0; j < 1000; j++) {
        res = repeatString("123", 100000);
        res = repeatStringSB("123", 100000);
    }

    time = System.nanoTime();
    res = repeatString("123", 1000000);
    time = System.nanoTime() - time;
    System.out.println("elapsed repeatString: " + time);

    time = System.nanoTime();
    res = repeatStringSB("123", 1000000);
    time = System.nanoTime() - time;
    System.out.println("elapsed repeatStringSB: " + time);

}

elapsed repeatString: 6006571
elapsed repeatStringSB: 9064937

 /**
   * Repeat a String as many times you need.
   *
   * @param i - Number of Repeating the String.
   * @param s - The String wich you want repeated.
   * @return The string n - times.
   */
  public static String repeate(int i, String s) {
    StringBuilder sb = new StringBuilder();
    for (int j = 0; j < i; j++)
      sb.append(s);
    return sb.toString();
  }

". ".repeat(7)  // Seven period-with-space pairs: . . . . . . . 

String str = "abc";
String repeated = str.repeat(3);
repeated.equals("abcabcabc");

/**
 * Returns a string whose value is the concatenation of this
 * string repeated {@code count} times.
 * <p>
 * If this string is empty or count is zero then the empty
 * string is returned.
 *
 * @param count number of times to repeat
 *
 * @return A string composed of this string repeated
 * {@code count} times or the empty string if this
 * string is empty or count is zero
 *
 * @throws IllegalArgumentException if the {@code count} is
 * negative.
 *
 * @since 11
 */ 

public static String rep(int a,String k)

       {
           if(a<=0)
                return "";
           else 
           {a--;
               return k+rep(a,k);
       }

Collections.nCopies( 3, "abc" ).stream().collect( Collectors.joining() );

static String repeat(String s, int length) {
    return s.length() >= length ? s.substring(0, length) : repeat(s + s, length);
}

for (int i = 0; i < 50; i++)
    System.out.println(repeat("_/‾\\", i));