Java) JDK / JVM / Runtime Data Areas

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What the JDK is made of

JDK: the development kit, which includes Java itself
JRE: the Java runtime environment

How it runs

Java is compiled into bytecode by javac, but the JVM then runs that bytecode through an interpreter. So Java is a compiled language that also carries the traits of an interpreted one.

JVM

Runtime Data Areas

Variables

Constant: final
Literal: something that is a value in and of itself

Character: char, ’‘
String: String, ”“

• ASCII
  • a 7-bit code that covers 128 character combinations
• Unicode
  • uses 16 bits — can represent up to 65,536 characters
• UTF-8
  • one of the encodings built on Unicode
  • uses 1 byte for letters/digits/symbols, and 3 bytes for Korean and Chinese characters
  • uses anywhere from 1 to 4 bytes to encode a single Unicode character

Type conversions

• Number to character: number + '0' -> character
• Character to number: character - '0' -> number
• Convert to string: variable + "" -> string
• String to number
  • Integer.parseInt("string")
  • Double.parseDouble("string")
• String to character
  • "string".charAt(idx)