Chapter 3 USING OPERATORS AND DECISION'S CONSTRUCTORS

3.1 USE JAVA'S OPERATORS

attribution operator, =, store a value to a variable;
byte < short < int < long < float < double;
char has the same size as byte but can't be negative;

Reference and attribution

attributions are ALWAYS by value copy, on primitive types we copy the value, on object reference we copy the reference (we do not duplicate the object);

Arithmetic operators

Used to do operations on numeric primitive variables: +, -, *, / and %;
The type of the result variable from the operation is the most embracing type or, at least, int;

Division by zero

Divide an integer by zero will result on a ArithmeticException;
Divide an float/double by zero will result on a positive/negative infinity;
NaN (Not a Number) is generated by the radiciation of some numbers and by some operations with infinity numbers;

Comparation operators

Equality operators: == and !=;
Order operators: >, <, >= and <=;
A comparation only returns true or false;
Numeric comparations ignores the value type;
References and boolean can only be compared with == or !=;
Can't compare incompatible types;
Operations with floating number are not always accurated;

Logic Operators

And &, Or |, Exclusive Or ^, Negation !;
Short circuit operators: && and ||;

Increment and Decrement operators

Pos Add 1 i++;
Pos Subtract 1 i--;
Pre Add 1 ++i;
Pre Subtract 1 --i;
Attributions and operations:
- i = i + 2 is the same as i += 2;
- i = i - 2 is the same as i -= 2;
- i = i * 2 is the same as i *= 2;
- i = i / 2 is the same as i /= 2;
- i = i % 2 is the same as i %= 2;
- On this cases the compilator allows you to do oparations with incompatible types. Ex: byte b1 = 3; b1 += 4;;

Ternary operator

boolean condiction ? if_true : if_false;
Should always return a value that can be used to an attribution;

Reference operators

.
Used to navegate on an object;

Strings concatenation

Can use the operator + to concatenate String;
String is the only class that accepts an operator besides .;
String concatenation with + is a syntax sugar, once the code is compiled it uses a StringBuilder;

Precedence

Order of execution:
1. pre increment/decrement
2. multiplications, divisions and mod;
3. subtraction addition;
4. shifts;
5. pos increment/decrement;

Casting of primitive types

double d = 0; float f = (float) d;
Casting with primitive types can result on precision lost;

Exercises

b
b
d
d
d
Linha 2, 3 e 4
b
d
e
Não compila.
c
c
a
f
c
f
b
c
a

3.2 USE PARENTHESES TO OVERRIDE OPERATORS PRECEDENCES

int a = 15 * 4 + 1; // 15 * 4 = 60, 60 + 1 = 61;
int b = 15 * (4 + 1); // 4 + 1 = 5, 15 * 5 = 75;

Exercises

a
b

3.3 Equality between Strings and other Objects

When using == on objects, it compares the object reference;
Two newly declared Strings are never equal (two distinct objects);
Strings are immutable;

String Pool

Java only uses string pool for literal types;
Strings created with the new operator does not go to pool;
when concatenating literal strings, the result string goes to the pool;
the result from concatenating a literal string and a new string object does not go into the pool;
the result from calling string methods are new Strings;
the result from calling string methods are not aqquired from pool;
when calling a method from a string resulting in the string itself, no new string is created;
"word" and "Word" are different, two Strings are added to the pool;

Equals

each Class has a .equals() method that can be overwritten;
new Integer(10) == new Integer(10) returns false;
new Integer(10).equals(new Integer(10)) returns true;
equals method must be public and receive an Object parameter;
equals method, by default, compares the object content;

Exercises

b
d
c
e
a

3.4 Utilizing If/Else Statement

if/else

It's all about flux control.
= for assignment and == for comparison, don't let yourself be fooled.
assignments inside an if statement must return true or false, otherwise: compilation error
braces are optional given the conditional is only one instruction long
else is optional when nothing is to be executed in case the statement returns false
we must have a statement to execute after an ´if´
there is no elseif, for this, if's are placed inside elses, ex: else if

Unreachable Code and Missing return

methods that return something, may not return something with a condition that returns something when true but lacks the else statement

Exercises

c
a
a
a
b

3.5 Using the Switch

Switch allowed arguments are int, wrapper smaller than Integer, String or Enum.
The switch variable must match the argument type
When using an int argumento and a double value in the case: compile error
The compiler casts the value of the argument if the case type is compatible
A case can be a literal, a final variable with a litteral value or expressions envolving both
Simple variables cannot be used as cases: compile error
null is not a valid case
To be considered a valid final variable, it must be declared as its initialized: final int num = 5;
The default case may appear in any order.
Breaks must be used between cases, otherwise there will be a cascade execution from the first matching case

Exercises

d
a
b
a
a
a

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