Stop asking me about strings in interviews

Stop asking me for strings in interviews

Strings are widely used in Java programming. In Java, strings are objects, and Java provides the String class to create and manipulate strings.

Introduction of the String

String definition:

public final class String
    implements java.io.Serializable.Comparable<String>, CharSequence {}
Copy the code

Why design immutable classes?

String is designed to be immutable for the following reasons: efficiency and security.

• Efficiency: 1. In earlier JVM implementations, methods modified by final were converted to inline calls to improve execution efficiency. Since Java SE5/6, this approach has been gradually abandoned. Therefore, in the current version of Java SE, there is no need to consider using final to improve method invocation efficiency. Make the method final only if you are sure that you do not want the method to be overridden. 2. Cache hashCode, String is immutable, so hashcode does not change, so cache is meaningful, do not need to recalculate.
• Security: String is often used as a parameter type for network connection, file operation, etc., which can have unexpected results if it can be changed.

Test mastery

In order not to waste your time, take a look at the following topics, if you are clear, you can skip this article.

public class Test {
    public static void main(String[] args) {
        String str1 = "HelloFlyapi";
        String str2 = "HelloFlyapi";
        String str3 = new String("HelloFlyapi");
        String str4 = "Hello";
        String str5 = "Flyapi";
        String str6 = "Hello" + "Flyapi";
        String str7 = str4 + str5;

        System.out.println("str1 == str2 result: " + (str1 == str2));

        System.out.println("str1 == str3 result: " + (str1 == str3));

        System.out.println("str1 == str6 result: " + (str1 == str6));

        System.out.println("str1 == str7 result: " + (str1 == str7));

        System.out.println("str1 == str7.intern() result: " + (str1 == str7.intern()));

        System.out.println("str3 == str3.intern() result: "+ (str3 == str3.intern())); }}Copy the code

String creation method

As you can see from the above, there are two ways to create a String:

Direct assignment

• This method creates objects in the string constant pool in the method area

  String str = "flyapi";
  Copy the code

The constructor

• This creates objects in heap memory

  String str = new String();
  Copy the code

Analysis of the

To understand String, you need to understand the stack, heap, and method areas in JVM memory. The brief picture is as follows:

• str1 == str2

  String str1 = "HelloFlyapi";
  String str2 = "HelloFlyapi";
  
  System.out.println(str1 == str2); // true
  Copy the code

When executing the first sentence, the JVM looks for the presence of the HelloFlyapi in the constant pool and returns a reference from the constant pool if it does. When it does not exist, an object is created in the character creation constant pool and a reference is returned.

When executing the second sentence, the same goes for returning the reference to the object created in the first sentence, since the first sentence has already been created in the constant pool.

• str1 == str3

  String str1 = "HelloFlyapi";
  String str3 = new String("HelloFlyapi");
  
  System.out.println(str1 == str3); // false
  Copy the code

Execute the first sentence as above.

When the second sentence is executed, an object is created in the heap, and an object is created in the constant pool when there is no ‘HelloFlyapi’ in the character creation constant pool; When a constant pool already exists, no new one is created.

• str1 == str6

  String str1 = "HelloFlyapi";
  String str6 = "Hello" + "Flyapi";
  
  System.out.println(str1 == str6); // true
  Copy the code

Since “Hello” and “Flyapi” are constants, the second sentence is automatically compiled to ‘String str6 = “HelloFlyapi”; ‘

• str1 == str7

  String str1 = "HelloFlyapi";
  String str4 = "Hello";
  String str5 = "Flyapi";
  String str7 = str4 + str5;
  
  System.out.println(str1 == str7); // false
  Copy the code

The first three variables store reference addresses in the constant pool.

When the fourth sentence is executed, the JVM creates a StringBuilder object based on STR4 in the heap, and then calls the StringBuilder append() method to merge with STR5. The toString() method is then called to create a String in the heap and assign a reference to the String to STR7.

Commonly used method

Here are the methods supported by the String class. For more details, see the Java String API documentation:

String related

Because of the immutability of strings, String changes were inefficient, and StringBuilder and StringBuffer appeared in later JDK versions.

• Used to choose

1. String is used when there are a few connections
2. Use StringBuilder when there is a large number of connection operations under a single thread
3. Use StringBuffer when there is a large number of connection operations in multiple threads

Common String interview questions

• String STR = new String(” ABC “) How many instances have been created?

This question is actually not rigorous, but the interview will generally encounter, so we want to add to explain.

Classes are loaded and executed separately: two are created

1. When the class is loaded, “ABC” is created and resides in the character creation constant pool (if not previously created in the load).
2. When this statement is executed, because the String instance corresponding to “ABC” already exists in the String constant pool, the JVM copies the instance to the heap and returns the reference address.

Bytecode we can see:

STR = new String(” ABC “)

Byte code:

    Code:
       0: new           #2 // class java/lang/String
       3: dup
       4: ldc           #3 // String abc
       6: invokespecial #4 // Method java/lang/String."
      
       ":(Ljava/lang/String;)
      
       9: astore_1
      10: return
Copy the code

Only one object is created at execution time (#2).

Questions about the interview, can see the answer: a large cattle rednaxelafx.iteye.com/blog/774673

I have arranged some interview documents and videos for you. The official account replies: Architect or interview video

This article is first published in the wechat public number: code combat