Java Programming I 2026

Project Jigsaw (Java9)

https://infoscis.github.io/2017/03/24/First-steps-with-java9-and-jigsaw-part-1/

https://infoscis.github.io/2017/03/24/First-steps-with-java9-and-jigsaw-part-2/

http://greatkim91.tistory.com/197

https://www.tutorialspoint.com/java/io/inputstream_read_byte_len.htm

The java.io.InputStream.read(byte[] b, int off, int len) method reads upto len bytes of data from the input stream into an array of bytes. If the parameter len is zero, then no bytes are read and 0 is returned; else there is an attempt to read at least one byte. If the stream is at the end of the file, the value returned is -1.

Parameters

• b − The destination byte array.
• off − The start offset in array b at which the data is written.
• len − The number of bytes to read.

Return Value

The method returns the total number of bytes read into the buffer, or -1 if there is no more data because the end of the stream has been reached.

Exception

• IOException − If an I/O error occurs.
• NullPointerException − If b is null.
• IndexOutOfBoundsException − If off is negative, len is negative, or len is greater than b.length – off.

http://blog.cleancoder.com/uncle-bob/2014/11/24/FPvsOO.html (OO versus FP)


// String immutable vs. StringBuilder mutable
// + operator (new StringBuilder(String.valueOf(str1)).append(str2).toString();
String a = "Hello";
String b = "Hello";
String c = a;
String d = "Hel" + "lo";
String e = "Hello" + "";
String f = a + ""; // String + StringLiteral => new String
String g = "";
String h = a + g; // String + String -> new String
String s = new String("Hello"); // new String
String t = new String("Hello"); // new String
String u = s + new String(""); // String + String -> new String
String v = s + new String(""); // String + String -> new String
String x = s.concat("~"); // concat creates new String
String y = s.concat("~"); // concat creates new String
System.out.println(getReference(a) + " a=" + a + " hashCode=" + a.hashCode());
System.out.println(getReference(b) + " b=" + b + " hashCode=" + b.hashCode());
System.out.println(getReference(c) + " c=" + c + " hashCode=" + c.hashCode());
System.out.println(getReference(d) + " d=" + d + " hashCode=" + d.hashCode());
System.out.println(getReference(e) + " e=" + e + " hashCode=" + e.hashCode());
System.out.println("a==e " + (a == e));
System.out.println(getReference(f) + " f=" + f + " hashCode=" + f.hashCode());
System.out.println("a==f " + (a == f));
System.out.println(getReference(g) + " g=" + g + " hashCode=" + g.hashCode());
System.out.println(getReference(h) + " h=" + h + " hashCode=" + h.hashCode());
System.out.println("a==h " + (a == h));
System.out.println(getReference(s) + " s=" + s + " hashCode=" + s.hashCode());
System.out.println("a==s " + (a == s));
System.out.println(getReference(t) + " t=" + t + " hashCode=" + t.hashCode());
System.out.println("s==t " + (s == t));
System.out.println(getReference(u) + " u=" + u + " hashCode=" + u.hashCode());
System.out.println(getReference(v) + " v=" + v + " hashCode=" + v.hashCode());
System.out.println("u==v " + (u == v));
System.out.println(getReference(x) + " x=" + x + " hashCode=" + x.hashCode());
System.out.println(getReference(y) + " y=" + y + " hashCode=" + y.hashCode());
System.out.println("x==y " + (x == y));

String str1 = “P”;
String str2 = “P”;
String str3 = str1 + str2;
String str4 = str1.concat(str2);
System.out.println(getReference(str1) + ” str1=” + str1 + ” hashCode=” + str1.hashCode());
System.out.println(getReference(str2) + ” str2=” + str2 + ” hashCode=” + str2.hashCode());
System.out.println(getReference(str3) + ” str3=” + str3 + ” hashCode=” + str3.hashCode());
System.out.println(getReference(str4) + ” str4=” + str4 + ” hashCode=” + str4.hashCode());
System.out.println(“str3==str4 ” + (str3 == str4));

java.lang.String@5a39699c a=Hello hashCode=69609650
java.lang.String@5a39699c b=Hello hashCode=69609650
java.lang.String@5a39699c c=Hello hashCode=69609650
java.lang.String@5a39699c d=Hello hashCode=69609650
java.lang.String@5a39699c e=Hello hashCode=69609650
a==e true
java.lang.String@3cb5cdba f=Hello hashCode=69609650
a==f false
java.lang.String@56cbfb61 g= hashCode=0
java.lang.String@1134affc h=Hello hashCode=69609650
a==h false
java.lang.String@d041cf s=Hello hashCode=69609650
a==s false
java.lang.String@129a8472 t=Hello hashCode=69609650
s==t false
java.lang.String@1b0375b3 u=Hello hashCode=69609650
java.lang.String@2f7c7260 v=Hello hashCode=69609650
u==v false
java.lang.String@2d209079 x=Hello~ hashCode=-2137068020
java.lang.String@6bdf28bb y=Hello~ hashCode=-2137068020
x==y false
java.lang.String@6b71769e str1=P hashCode=80
java.lang.String@6b71769e str2=P hashCode=80
java.lang.String@2752f6e2 str3=PP hashCode=2560
java.lang.String@e580929 str4=PP hashCode=2560
str3==str4 false

The + operator produces a new String by appending the second operand onto the end of the first operand.

How to Concatenate Strings in Java | Developer.com

Strings and Object References in Java: The concat() Method | Saylor Academy

Java :: 연산자 (Double Colon Operator)는 람다식을 대체하여 메서드 참조(method reference)로 사용된다. 즉, 람다식이 사용될 수 있는 Functional Interface Implementation에서만 사용 가능하다.

• • Using Lambda

    Comparator<Person> c = (Person p1, Person p2) ->
                        p1.getName().compareTo(p2.getName());  
    

• • Using Lambda with type interference

    Comparator<Person> c = (p1, p2) -> p1.getName().CompareTo(p2.getName());

  • Using method reference ( :: operator)

    Comparator<Person> c = Comparator.comparing(Person::getName);

  • Method reference  ( :: operator)

    Function<Person, String> getName = Person::getName; // Person->String
    String name = getName.apply(person1); 
    

  • Method reference ( :: operator)는 람다식과 동일한 처리를 하는 식이지만 메소드 본문을 제공하는 대신 기존 메소드를 이름으로 참조한다.

    Function<Double, Double> sq = (Double x) -> x * x; // lambda 
    double result = sq.apply(3); // 3 * 3 = 9
     
    Function<Double, Double> sq2 = MyClass::square; // MyClass에 정의된 square 정적 메소드
    double result2 = sq2.apply(3); // 3 * 3 = 9

    BiFunction<Double, Double, Double> add = (x, y) -> x + y; // lambda
    double result = add.apply(3.1, 3.2); // 3.1 + 3.2 = 6.3
    
    BiFunction<Double, Double, Double> add2 = MyClass::sum; // MyClass에 정의된 sum 정적 메소드 
    double result2 = add2.apply(3.1, 3.2); // 3.1 + 3.2 = 6.3