# 1-C: Data Types & Operations
## Learning Targets
* I can declare variables of common Java data types and assign values.
* I can control the flow of my app with loops, conditionals and method calls.
* I can write efficient conditionals and return comparisons in my methods.
* I can describe the difference between logic, run time and compile time errors.
## Declaring Variables
Primitives are objects that live entirely in stack memory (they're lowercase like **`int`** and not like **`String`**) . We'll go over the difference between stack and heap memory later. For now, just think of stack memory as the lightweight stuff that's really fast-access. Take a look at the [types of Java primitives](https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html).
### Naming Conventions
`CompoundCapitalize` name of classes
`camelCase` variables and methods
`ALLCAPS` variables marked as *`final`*, meaning they can't change
`snake_case` is typically used only in Python and I miss it every time I work in Java or C#
## Operators
### Arithmetic
| Operator | Description |
Example (a=10, b=20)
|
| ------------------- | -------------------------------------------------------------------------- | -------------------------------- |
| + (Addition) | Adds values on either side of the operator. | a + b will give 30 |
| - (Subtraction) | Subtracts right-hand operand from left-hand operand. | a - b will give -10 |
| \* (Multiplication) | Multiplies values on either side of the operator. | a \* b will give 200 |
| / (Division) | Divides left-hand operand by right-hand operand. | b / a will give 2 |
| % (Modulus) | Divides left-hand operand by right-hand operand and returns the remainder. | b % a will give 0 |
| ++ (Increment) | Increases the value of operand by 1. | b++ gives 21 |
| -- (Decrement) | Decreases the value of operand by 1. | b-- gives 19 |
#### Integer Math
What's `9 / 4`? In Java, it's just`2`, but if you wrote it like `9.0 / 2`, then the answer would be 2.5. If you don't introduce a decimal to the operation, if you only give Java literal `int`s , it'll match that format and ignore decimals.
{% embed url="" %}
### Comparison
We evaluate / test data and produce a `true` or `false` value.
| Operator | Description | Example (a=10, b=15) | Result |
| -------- | ------------------------ | -------------------- | ------ |
| == | Equality operator | a==b | false |
| != | Not Equal to operator | a!=b | true |
| > | Greater than | a>b | false |
| < | Less than | a\= | Greater than or equal to | a>=b | false |
| <= | Less than or equal to | a<=b | true |
#### Short-Circuit Evaluation
## Arrays
You can declare and initialize an int like `int x = 5;` Similarly, you can create an array that contains three ints like this: `int[] x = {5, 10, 15};`
{% embed url="" %}
{% hint style="info" %}
Arrays are **immutable**. They don't change in size. Java uses a fancier object called an `ArrayList` that is more flexible. So if one of your friends is trying to convince you to increase the size of an array, just say no.
{% endhint %}
### Array Tricks
```java
// Let's demonstrate some array skills!
// Here's a blank array that's got spots for five ints
int[] arr = new int[5];
// You can also create an array with initial values
int[] x = {5, 10, 15};
```
#### How long is that array?
`x.length; // note: this isn't a method like it is in the String class.`
#### What's the first element in the array?
`x[0]; // this will print "5" in my example`
#### What's the last element in the array?
`x[x.length-1]; // this will print "15" in my example`
#### What other skills do you need to know?
* Loop through every element in the array
* Check if an element is found in an array
* Make an array of other types of objects
## Method Calls
This is a part of **control flow**. The thread, the computer's train of thought, is being sent over to work through a set of commands called a method or function (either name works in this class).
The main function is calling max(), passing it a few numbers and saving the result
The variable declared in the header is scoped to that function
