Java basics

JDK components

Java development kit - javac - compiler, java - runs bytecode on JVM, javadoc - generates documentation, jar - packages Java applications.

method signiture

A method signature consists of the method's name and its parameter list(only the parameter types count, not the parameter names), which defines how the method can be called.

classpath

Tells the JVM where to look for classes and packages. The paths are on the local machine.

Instance initializer

Code block without method name, runs before constructor.

Object vs reference

Object - the actual values stored in the heap, can be accessed only through the reference. Reference - the variable that can be used to access the object - stores the address of the object

inboxing vs unboxing

inboxing: primitive → object, can parse string; unboxing: obj → primitive; e. g. Double obj = Double.valueOf(“2”); double primitive = obj.doubleValue(); // now java can make the cast implicitly: autoboxing, autounboxing

What kind of variables are initialized automatically?

Instance and class variables are initialized by default value.

Numeric promotion rule

When we use binary arithmetic operators(+,-, *, /, %) → smaller data types will be promoted to lager data types (short→ int)

Switch case classic syntax vs new syntax

switch(value) {

case v1:

//code

break;

default:

//code

}

String dayName = switch (day) {

case 1 -> "Monday"; // simple expression

case 2 -> { // block with multiple statements

System.out.println("It's Tuesday!");

yield "Tuesday"; // yield returns the value for this case

}

case 3 -> "Wednesday";

default -> "Other day";

};

for each enhanced loop syntax

for (ElementType element : collection) {

// use element, but cannot modify collection, element is a copy;

}

varargs

variable arguments; notation: arg…; It must be only one vararg parameter, and it must be the last parameter in the param. list. - the function can be called with empty paranthesis or with null: func(), func(null);

Maven project structure, what happens after compilation, how to reach a resource?

the compiled java files go to target/build, with the package structure(after the java package);

the resource folders contents go directly into target/build

reach resource(from compiled code): var stream = CurrentClass.class.getClassLoader().getResourceAsStream(‘filename‘);

classloader always looks for absolute path from the target/build

CurrentClass.class.getResourceAsStream(‘/filename‘); - i need to add the ‘/’ to look from target/build, or else it will search inside the package the class is

What is classloader?

• locates classes/resources

• gives back stream to process resource

• loads class by reading it and creating a class object in memory.

• the class object contains data about the class structure itself

• Class <MyClass> clazz = Myclass.class; → reflection