Expressions, variables, operators

Expressions in the Java language (basic sentences)

int x, y ;  / / declare
int z = x + y; / / operation
if(true) {.. ; }/ / determine
while(true) {.. ; }/ / loop
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• Classification of sentences
  • The statement
  • operation
  • judge
  • cycle

Java language variables

What does a variable mean

• Properties of objects are similar to, but not identical to, variables
• What types do variables have
  • Instance variables (non-static properties of the class)
  • Class variables (static properties of a class)
  • Local variables (temporary properties declared inside methods)
  • Method parameters (properties specified when declaring methods)
• When do you call it a property and when do you call it a variable
  • When you talk about attributes, you don’t include local variables and parameters
  • When you talk about variables, you can mean class properties, object properties, local variables, method parameters

Naming rules for variables

• Case sensitivity
  • Variables declared as Car and declared as Car are two completely different variables
• Unlimited length
  • Consists of Unicode characters and digits and must start with a character
    • It can be declared as Abc123, but not as 1abc23
    • Do not start with $or _
    • Except for the first character, there is no requirement for subsequent characters and digits
    • When declaring variables with only one word, use all lowercase
      • Such as the car
    • When declaring a variable with two or more words, the first character of the second word is capitalized
      • Such as bigCar

Java language operator