[JAVA] Various methods of the String class


I am studying for the Java SE 8 Silver exam, and I would like to summarize what I learned during that time.

This time, I will organize various methods of the String class.

(I regret that there are many articles on character string manipulation ... sweat)

Method type

I have summarized the main methods that appear in the exam.

Method name Description
char charAt(int i) Returns the character at position i.
String concat(String str) Add a string
boolean endsWith(String suffix) Returns true if the string ends with the specified string, false if not.
boolean equalsIgnoreCase(String str) Compare strings. Insensitive to case.
int indexOf(int ch) Returns the position where the value entered in the actual argument first appears.
int lastIndexOf(int ch) Returns the position where the value entered in the actual argument appears last.
int length() Returns the number of characters in the string.
String replace(char o, char n) Returns the result of replacing the character o in the string with the character n.
boolean startsWith(String prefix) Returns true if the string starts with the value you put in the actual argument.
String substring(int i) 0 ~Returns a substring up to the value entered in the actual argument. (You can specify the start point.)
String toLowerCase() Convert uppercase to lowercase
String toUpperCase() Convert lowercase letters to uppercase
String toString() Returns the string held by the object.
String trim() Remove whitespace. (However, full-width space symbols are not removed)

Operation check

I'll delve into the methods that I find difficult to understand.

-Substring () method

Regarding the method of extracting the character string, if you do not pay attention to how to specify the range, the output will not be as expected.

Below is the sample code.


public class Main {

	public static void main(String[] args) {

		String str = "There was a pig kimchi";

		//Explanation ①
		System.out.println(str.substring(0, 5));



Output result


Pig kimchi

Analysis ① Only "Pig Kimchi" is taken out, but when taking out ***, there must be 0 before the first character ***

Please be careful. It can be taken out by showing the range from 0 to 5.

Buta Kimchi is Ah, Ma.
 ↑    ↑    ↑    ↑    ↑    ↑    . . . .
 0    1   2   3   4   5

Another thing I want to organize is the trim () method.

・ Trim () method

It is a method that removes spaces before and after the character string.

It also removes tab characters such as \ t, \ n or \ r, and newlines.

Please note that it does not remove double-byte spaces and does not remove whitespace in the string.

To take the exam

There are many problems that can be understood by solving Java Silver problems.

Even though I called a convenient method of the String class and fetched the value

Since it is not assigned, there are many nasty problems such as the output is the same as before calling the method.

Therefore, I would like to take on the challenge with plenty of time when taking the exam.

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