API : Application Programming Interface
UI : User Interface
api documention java 검색
java.lang : Math가 포함된 패키지package,class,method
public class ClassApp {
public static void main(String[] args) {
System.out.println(Math.PI);
Math.floor(1.6);
Math.ceil(1.4);
}
}
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.PrintWriter;
public class InstanceApp {
public static void main(String[] args) throws IOException {
PrintWriter p1 = new PrintWriter("result1.txt");
p1.write("Hello 1");
p1.close();
PrintWriter p2 = new PrintWriter("result2.txt");
p2.write("Hello 2");
p2.close();
}
}
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.PrintWriter;
public class InstanceApp {
public static void main(String[] args) throws IOException {
PrintWriter p1 = new PrintWriter("result1.txt"); //p1 =인스턴스
//하나의 클래스를 new를 사용해서 인스턴스를 만들어서
//계속 사용할 수 있게 만듬
p1.write("Hello 1");
p1.close();
PrintWriter p2 = new PrintWriter("result2.txt");
p2.write("Hello 2");
p2.close();
}
}
constructor 가 있다면 인스턴스 만드는것이 허용되어있다는 것