Java 8 `StringJoiner`

原文： https://beginnersbook.com/2017/10/java-8-stringjoiner/

在 java 8 中，java.util包中引入了一个新类StringJoiner。使用这个类，我们可以使用指定的分隔符连接多个字符串，我们还可以在连接多个字符串时为最终字符串提供前缀和后缀。在本教程中，我们将看到几个StringJoiner类的示例，在本指南的最后，我们将看到StringJoiner类的方法。

Java StringJoiner示例 1：通过指定分隔符来连接字符串

在这个例子中，我们使用StringJoiner连接多个字符串。在创建StringJoiner的实例时，我们将分隔符指定为连字符（-）。

import java.util.StringJoiner;  
public class Example {  
    public static void main(String[] args) {  
        // Passing Hyphen(-) as delimiter
        StringJoiner mystring = new StringJoiner("-");    
        // Joining multiple strings by using add() method  
        mystring.add("Logan");  
        mystring.add("Magneto");  
        mystring.add("Rogue");  
        mystring.add("Storm");  
        // Displaying the output String
        System.out.println(mystring);  
    }  
}

输出：

Logan-Magneto-Rogue-Storm

Java StringJoiner示例 2：为输出字符串添加前缀和后缀

import java.util.StringJoiner;  
public class Example {  
    public static void main(String[] args) {  
        /* Passing comma(,) as delimiter and opening bracket
         * "(" as prefix and closing bracket ")" as suffix
         */
        StringJoiner mystring = new StringJoiner(",", "(", ")");    
        // Joining multiple strings by using add() method  
        mystring.add("Negan");  
        mystring.add("Rick");  
        mystring.add("Maggie");  
        mystring.add("Daryl");  
        // Displaying the output String
        System.out.println(mystring);  
    }  
}

输出：

(Negan,Rick,Maggie,Daryl)

StringJoiner示例 3：合并两个StringJoiner对象

import java.util.StringJoiner;  
public class Example {  
   public static void main(String[] args) {  
    /* Passing comma(,) as delimiter and opening bracket
     * "(" as prefix and closing bracket ")" as suffix
     */
    StringJoiner mystring = new StringJoiner(",", "(", ")");    
    mystring.add("Negan");  
    mystring.add("Rick");  
    mystring.add("Maggie");  
    mystring.add("Daryl");  
    System.out.println("First String: "+mystring);
    /* Passing hyphen(-) as delimiter and string "pre"
     * as prefix and string "suff" as suffix
     */
    StringJoiner myanotherstring = new StringJoiner("-", "pre", "suff");    
    myanotherstring.add("Sansa");  
    myanotherstring.add("Imp");  
    myanotherstring.add("Jon");  
    myanotherstring.add("Ned"); 
    System.out.println("Second String: "+myanotherstring);
    /* Merging both the strings  
     * The important point to note here is that the output string will be 
     * having the delimiter prefix and suffix of the first string (the string
     * which is calling the merge method of StringJoiner)
     */
    StringJoiner mergedString = mystring.merge(myanotherstring);   
    System.out.println(mergedString);  
   }  
}

输出：

First String: (Negan,Rick,Maggie,Daryl)
Second String: preSansa-Imp-Jon-Nedsuff
(Negan,Rick,Maggie,Daryl,Sansa-Imp-Jon-Ned)

在上面的例子中，我们已经看到了StringJoiner类的add()和merge()方法。让我们看看这个类的其他方法。

StringJoiner示例：setEmptyValue()，length()和toString()方法

import java.util.StringJoiner;  
public class Example {  
    public static void main(String[] args) {  
        //Comma(,) as delimiter
        StringJoiner mystring = new StringJoiner(",");   
        /* Using setEmptyValue() method, we can set the default value
         * of a StringJoiner instance, so if the StringJoiner is empty
         * and we print the value of it, this default value will be
         * displayed
         */
        mystring.setEmptyValue("This is a default String");  
        /* We have not added any string to StringJoiner yet so
         * this should display the default value of StringJoiner
         */
        System.out.println("Default String: "+mystring);  
        // Adding strings to StringJoiner  
        mystring.add("Apple");  
        mystring.add("Banana"); 
        mystring.add("Orange");
        mystring.add("Kiwi");
        mystring.add("Grapes");
        System.out.println(mystring);  
        /* The length() method of StringJoiner class returns the 
         * length of the string (the number of characters in the 
         * StringJoiner instance)
         */
        int length = mystring.length();  
        System.out.println("Length of the StringJoiner: "+length);  
        /* The toString() method is used for converting a StringJoiner
         *  instance to a String. 
         */
        String s = mystring.toString();  
        System.out.println(s);   
    }  
}

输出：

Default String: This is a default String
Apple,Banana,Orange,Kiwi,Grapes
Length of the StringJoiner: 31
Apple,Banana,Orange,Kiwi,Grapes

参考文献：

Java 8 - StringJoiner JavaDoc