Java Palindrome Tutorials

Posted 2020-12-14 PrimerPlus

Java Palindrome tutorial shows how to work with palindromes in Java.

Java Palindrome教程展示了如何在Java中使用回文

Palindrome is a word, number, phrase, or other sequence of characters which reads the same backward as forward, such as madam or racecar.

回文报是一个单词，数字，短语或其它字符序列，它们的向前和向后读取相同，例如madam或racecar

In this tutorial we show several ways to check if a string is a palidrome in Java.

在本教程中，展示了几种检查字符串是否是Java回文的方法

Java Palindrome with StringBuilder

The StringBuilder‘s reverse method causes this character sequence to be replaced by the reverse of the sequence.

package com.zetcode.palindrome;

public class Palindrome1 {

    public static void main(String[] args) {

        System.out.println(isPalindrome("radar"));;
        System.out.println(isPalindrome("kayak"));
        System.out.println(isPalindrome("forest"));;
    }

    private static boolean isPalindrome(String original) {

        String reversed = new StringBuilder(original).reverse().toString();
        return original.equals(reversed);
    }
}

In the example, we pass the original string to the StringBuilder and reverse it. Then we compare the reversed string to the original with equals.

在示例中，我们将原始字符串传递给StringBuilder并将其反转。然后，我们将颠倒的字符串与原始字符串进行比较

Java Palindrome with for loop and charAt

具有for循环和charAt的Java回文

The String‘s charAt method returns the char value at the specified index. An index ranges from 0 to length() - 1.

字符串的charAt方法返回指定索引处的char值。索引范围是0到length()-1

package com.zetcode.palindrome;

public class Palindrome2 {

    public static void main(String[] args) {

        System.out.println(isPalindrome("radar"));
        System.out.println(isPalindrome("kayak"));
        System.out.println(isPalindrome("forest"));
    }

    private static boolean isPalindrome(String original) {

        String reversed = "";

        int len = original.length();

        for (int i = len - 1; i >= 0; i--) {

            reversed = reversed + original.charAt(i);
        }

        return original.equals(reversed);
    }
}

In the example, we build the reversed string with a for loop that goes from the end of the original string. We get the current character with charAt.

在示例中，我们使用从原始字符串末尾开始的for循环构建反向字符串。通过charAt获得当前字符

int len = original.length();

We get the number of characters in the original string with length.

我们获得原始字符串中具有的字符数长度

for (int i = len - 1; i >= 0; i--) {
        
    reversed = reversed + original.charAt(i);
}

We build the reversed string by taking the characters from the end of the original string with charAt.

我们通过使用charAt从原始字符串的末尾获取字符来构建反向字符串

Java Palindrome with toCharArray

Java回文与toCharArray

The String‘s toCharArray converts the string to a new character array.

toCharArray将字符串转换为新的字符数组

package com.zetcode.palindrome;

public class Palindrome3 {

    public static void main(String[] args) {

        System.out.println(isPalindrome("radar"));
        System.out.println(isPalindrome("kayak"));
        System.out.println(isPalindrome("foreat"));
    }

    private static boolean isPalindrome(String original) {

        char[] data = original.toCharArray();

        int i = 0;
        int j = data.length - 1;

        while (j > i) {
            if (data[i] != data[j]) {
                return false;
            }
            ++i;
            --j;
        }
        return true;
    }
}

In the example, we turn the original string to an array of characters. In a while loop, we start to compare the characters from the both sides of the string; starting with the leftmost and rightmost characters. We go until the middle of the string.

在示例中，我们将原始字符串转换为字符数组。在while循环中，我们开始比较字符串两侧的字符；从最左边和最右边的字符开始, 一直走到字符串的中间

Java Palindrome with Stack

The Stack is a last-in-first-out (LIFO) collection.

package com.zetcode.palindrome;

import java.util.Stack;

public class Palindrome4 {

    public static void main(String[] args) {

        System.out.println(isPalindrome("radar"));
        System.out.println(isPalindrome("kayak"));
        System.out.println(isPalindrome("foreat"));
    }

    private static boolean isPalindrome(String original) {

        char[] data = original.toCharArray();

        Stack<Character> stack = new Stack<>();

        for (char c : data) {
            stack.push(c);
        }

        char[] data2 = new char[data.length];

        int len = stack.size();
        
        for (int i = 0; i < len; i++) {
            data2[i] = stack.pop();
        }

        String reversed = new String(data2);

        return original.equals(reversed);
    }
}

The example uses the Java Stack container to build a reversed string.

该示例使用Java Stack容器构建反向字符串

char[] data = original.toCharArray();

First, we turn the string to an array of characters with toCharArray.

首先，使用toCharArray将字符串转换为字符数组

Stack<Character> stack = new Stack<>();

for (char c: data) {

    stack.push(c);
}

In the second step, we push the characters to the stack.

第二步，将字符推入堆栈

char[] data2 = new char[data.length];

This array will hold the reversed characters.

该数组将包含反向字符

for (int i = 0; i < len; i++) {

    data2[i] = stack.pop();
}

We get the reversed string now by popping the characters from the stack.

现在通过从堆栈中弹出字符来获得反向字符串

String reversed = new String(data2);
    
return original.equals(reversed);

We create the reversed string from the array and compare it with the original string using equals.

从数组创建反向字符串，然后使用equals将其与原始字符串进行比较

In this tutorial, we have check if a string is a palindrome. We have created various algorithms to check a palindrome.

在本教程中，我们检查字符串是否是回文。并创建了各种算法来检查回文