String

✅ String is reference

• String is reference value
• can create String as literal or as instance

String str1= "hello";  //literal
String str2 = new String("hello"); //instance

• 🟰 two code is same

✅ String and equals()

• ==: compare memory
• equals(): compare value

• for comparing String, use equals()

• ✔️ String as instance

public static void main(String[] args) {
    String str1 = new String("hello");
    String str2 = new String("hello");

    System.out.println(str1 == str2); //false, different memory
    System.out.println(str1.equals(str2)); //true
}

• ==: false

• equals(): true

• ❗️ String as literal

public static void main(String[] args) {
    String str1 = "hello";
    String str2 = "hello";

    System.out.println(str1 == str2); //true
    System.out.println(str1.equals(str2)); //true
}

• ==: true

• equals(): true

• if String is literal, Java tries to use memory efficiently
• Java has String Pool saved in heap
• if same literal exists in String Pool, does not create again
• thus, str1, str2 has SAME memory value
• 👍🏻 save memory for same string literal value

✅ String is immutable

• once String value is set, CAN NOT change
• immutable
• if want to change, CREATE NEW STRING

//⭐️ String is immutable
String str = "hello";
str.concat("Java"); //try to add "Java"
System.out.println(str); //return hello, does not change

//⭐️ If want to change immutable string, create new String instance
String newStr = str.concat("Java");  //add "Java"
System.out.println(newStr); //return helloJava

✅ Stringbuilder

• used to change String without creating new string instance

• NOT immutable

• 👎🏻 String is immutable side effect
• 👎🏻 everytime you want to change String, need to create a new string instance

String str = "A" + "B" + "C" + "D"
//to do this need to create A, AB, ABC, ABCD
// 👎🏻 do not need A, AB, ABC

• 👎🏻 not efficient memory use
• 💊 string builder

• ✔️ StringBuilder is NOT immutable
• can change without creating new String instance
• 👍🏻 String을 자주 바꿀거라면 String Builder을 사용하는게 더 효과적

StringBuilder sb = new StringBuilder();
sb.append("A");
sb.append("B");
sb.append("C");
sb.append("D");  //ABCD

sb.insert(4, "Java"); //ABCDJava

sb.delete(4, 8); //ABCD

sb.reverse(); //DCBA

String result = sb.toString() //이제 더 안 바꿀꺼면 toString으로 string으로 바꾸기

• ✔️ When is StringBuilder used?
• loops

• changing a long String many many times

• 🆚 StringBuffer: synchronization, in multi-thread, but slower
• StringBuilder: can run at same time, might have problems in multi-thread, but faster

✅ StringBuilder and Method Chaining

• ✔️ Method Chaining
• chain methods into one line

ValueAdder adder = new ValueAdder();
adder.add(1);
adder.add(2);
adder.add(3);
int result = adder.getValue();

//method chaining
ValueAdder adder = new ValueAdder();
int result = adder.add(1).add(2).add(3).getValue();

• ✔️ StringBuilder supports method chaining

StringBuilder sb = new StringBuilder();
sb.append("A");
sb.append("B");
sb.append("C");
sb.append("D");

//method chaining
sb.append("A").append("B").append("C").append("D");

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