Batch splice string

Two ways

Common strings are spliced into a single string , Use it directly “+”

from JDK5 Start ,Java The compiler is optimized , Use “+” String concatenation , After compiling, the compiler will automatically optimize to use StringBuilder

Create using “+” String concatenation And use StringBuilder String concatenation Methods ; and newly added Junit The test case , , respectively, Call the concatenation string 100000 Time （ here Not a circular splice , Instead, perform multiple splices , Because one time splicing takes too little time , No difference can be seen ）, Printing time .

 /** *  Use + String concatenation  */
 public String concatenationStringByPlus(String prefix, int i) {
    
     return prefix + "-" + i;
 }

 /** *  Use StringBuilder String concatenation  */
 public String concatenationStringByStringBuilder(String prefix, int i) {
    
     return new StringBuilder().append(prefix).append("-").append(i).toString();
 }

 /** *  Test use + Splicing strings takes time  */
 @Test
 public void testStringConcatenation01ByPlus() {
    
     long startTime = System.currentTimeMillis();
     int count = 100000;
     for (int i = 0; i < count; i++) {
    
         String str = concatenationStringByPlus("testStringConcatenation01ByStringBuilder:", i);
     }
     long endTime = System.currentTimeMillis();
     System.out.println("testStringConcatenation01ByPlus, String concatenation " + count + " Time , cost " + (endTime - startTime) + " second ");
 }


 /** *  Test use StringBuilder Splicing strings takes time  */
 @Test
 public void testStringConcatenation02ByStringBuilder() {
    
     long startTime = System.currentTimeMillis();
     int count = 100000;
     for (int i = 0; i < count; i++) {
    
         String str = concatenationStringByStringBuilder("testStringConcatenation02ByStringBuilder:", i);
     }
     long endTime = System.currentTimeMillis();
     System.out.println("testStringConcatenation02ByStringBuilder, String concatenation " + count + " Time , cost " + (endTime - startTime) + " second ");
 }

testStringConcatenation01ByPlus, String concatenation 100000 Time , cost 33 second 
testStringConcatenation02ByStringBuilder, String concatenation 100000 Time , cost 36 second 

To class File directory , perform javap -c StringTest.class, Yes class Decompile files , see Compiled code differences . Don't use it here Intellij idea and JD decompile , Because decompilation has optimization , Will be decompiled into “+” Spliced , I can't see the real situation after compilation .

It can be seen that The two splicing methods are exactly the same after decompiling , There is no difference , The same goes for execution efficiency

Loop concatenates a string , Use StringBuilder

although “+” The concatenated string will also become StringBuilder, however Each cycle of processing will new One StringBuilder object , Time will be greatly increased . and Use it directly StringBuilder,new Just once , It's relatively efficient

/** *  Recycling + String concatenation  */
@Test
public void testLoopStringConcatenation03ByPlus() {
    
    long startTime = System.currentTimeMillis();
    int count = 10000;
    String str = "testLoopStringConcatenation03ByPlus:";
    for (int i = 0; i < count; i++) {
    
        str = str + "-" + i;
    }
    System.out.println(str);
    long endTime = System.currentTimeMillis();
    System.out.println("testLoopStringConcatenation03ByPlus, String concatenation " + count + " Time , cost " + (endTime - startTime) + " second ");
}

/** *  Test recycling StringBuilder Splicing strings takes time  */
@Test
public void testLoopStringConcatenation04ByStringBuilder() {
    
    long startTime = System.currentTimeMillis();
    int count = 100000;
    StringBuilder stringBuilder = new StringBuilder("testLoopStringConcatenation04ByStringBuilder:");
    for (int i = 0; i < count; i++) {
    
        stringBuilder.append("-");
        stringBuilder.append(i);
    }
    String str = stringBuilder.toString();
    System.out.println(str);
    long endTime = System.currentTimeMillis();
    System.out.println("testLoopStringConcatenation04ByStringBuilder, String concatenation " + count + " Time , cost " + (endTime - startTime) + " second ");
}

testLoopStringConcatenation03ByPlus, String concatenation 10000 Time , cost 463 second 
testLoopStringConcatenation04ByStringBuilder, String concatenation 10000 Time , cost 13 second 

summary

 Use... For circular splicing “+” String splicing is inefficient , Recommended StringBuilder String concatenation .