Java lesson 2: Variable Boxes in Your Inventory

WHAT YOU WILL MAKE

Create a changing console player status bar

The player name in the welcome screen is always written as Alex. If another player comes, every name must be found again. Variables give us a better way: put data in a named location, read it, and update it. What we need to understand in this lesson is not only "how to write variables", but also "what exactly does it save at this moment".

Before you start: It is recommended to complete Lesson 1 first

01 / 10

tag helps us find the current value

Storage rooms rely on labels to find items; programs rely on variable names to read data.

Steve put the bread into the unlabeled box, but opened the cobblestone box during dinner time. Variable names solve a similar problem: letting the program find the data it needs. It is not the data itself, but the name used to access the data.

Let’s start with two types: int holds integers, and String holds text. Health points and item quantities are suitable for integers; player names are suitable for text. More types will be discussed in the next lesson.

02 / 10

Statement, initialization, reading

Read int health = 20;

intType · Save integer
healthVariable name · Find the current value
=Assignment number · Write the value on the right
20Initial value · The value of the first deposit
;Semicolon · End this statement

Complete program · PlayerStatus.java

public class PlayerStatus {
    public static void main(String[] args) {
        String playerName = "Alex";
        int health = 20;
        int bread = 3;
        System.out.println(playerName + " Life=" + health);
        System.out.println("Bread.=" + bread);
    }
}

Output

Alex Life=20
Bread.=3

When health is not quoted, the value of the variable is read; after "health" is quoted, it is just a fixed text. Change the player name to Steve and the bread to 5. Guess the two lines of output first and then run it.

03 / 10

Assignment: Calculate the right side first, then write the left side

Assignment is like a transfer: prepare the result first, and then put it back at the designated location.

three actions of bread = bread - 1

Read Old Valuesbread is 3
Calculate the right side3 - 1 gets 2
Write back to the leftbread becomes 2

Snippet: place inside main and run separately

int bread = 3;
System.out.println("Before eating=" + bread);
bread = bread - 1;
System.out.println("After eating=" + bread);
int diamonds = 0;
diamonds = diamonds + 3;
System.out.println("diamond=" + diamonds);

Output

Before eating=3
After eating=2
diamond=3

04 / 10

One output is a snapshot

Name is not equal to content: four current data

Variable name	Type	Current value
playerName	String	"Alex"
health	int	20
bread	int	3
diamondCount	int	0

The line of text previously output will not change with the variable. If you want to show the new status, you need to output it again. When the program is re-run, the initialization statements are also re-executed.

Snippet: place inside main and run separately

int chestA = 5;
int chestB = chestA;
chestA = 9;
System.out.println("A=" + chestA);
System.out.println("B=" + chestB);

Output

A=9
B=5

05 / 10

Update health values along the timeline

The same variable, four moments

Birthhealth = 20
Injuryhealth - 4
Get 16
Eating.health + 3
Get 19
Restore initial valuehealth = 20

Snippet: place inside main and run separately

int health = 20;
System.out.println("Birth=" + health);
health = health - 4;
System.out.println("Injury=" + health);
health = health + 3;
System.out.println("Eating.=" + health);
health = 20;
System.out.println("Restore initial value=" + health);
int arrows = 10;
arrows += 5;
arrows -= 2;
System.out.println("Arrow=" + arrows);

Output

Birth=20
Injury=16
Eating.=19
Restore initial value=20
Arrow=13

06 / 10

Good names let the code explain itself

Do a check on named street signs

Example	Judgment
playerName	Clear, use camelCase
diamondCount	Explain that this is the number of diamonds
2player	Error: cannot start with a number
player health	Error: Name cannot have spaces
class	Error: This is a Java keyword
x	Can run, but the purpose is unclear

When starting to learn, it is recommended to start with a lowercase English letter, and use camelCase for multiple words: the first word is lowercase, and the first letter of subsequent words is capitalized. Java is case-sensitive, health and Health are not the same name.

Snippet: place inside main and run separately

int playerLevel = 8;
int torchCount = 24;
String petName = "Snow";
System.out.println(playerLevel + " / " + torchCount);
System.out.println(petName);

Output

8 / 24
Snow

07 / 10

Variable troubleshooting: first distinguish what kind of problem it is

Examining a cause and then observing the result is more effective than tearing down the entire building.

Deliberate typo: three different problems

int 2player = 10;
int bread = 3;
int bread = 2;
int diamonds;
System.out.println(diamonds);

2player starts with a number and changes to playerLevel.
Repeated declaration of bread in the same scope: remove the second int when updating.
Local variable diamonds has no value yet: explicitly assign value before reading.

Snippet: place inside main and run separately

int playerLevel = 10;
int bread = 3;
bread = 2;
int diamonds = 0;
System.out.println(playerLevel + " / " + bread + " / " + diamonds);

Output

10 / 2 / 0

08 / 10

Design a changing player status bar

In the same adventure, events change data; new output records the changed state.

First draw the working route of the status bar

Create variablesname, life
Bread, diamonds
Birth SnapshotFirst output the initial value
Event UpdateInjury, supply
With mining
Output againCheck new value

This work does not require input, looping or conditional judgment. Just use the declarations, assignments, and outputs you've already learned to observe data changing with events. Before each update, write down the expected values on paper.

Birth: 20 lives, 3 bread, 0 diamonds.
Injured: Life minus 4, others unchanged.
Supply and mining: life plus 3, bread minus 1, and diamonds plus 3.

09 / 10

Complete work and output verification

Complete program · AdventureStatus.java

public class AdventureStatus {
    public static void main(String[] args) {
        String playerName = "Alex";
        int health = 20;
        int bread = 3;
        int diamondCount = 0;
        System.out.println("Birth | " + playerName
            + " | Life=" + health + " | Bread.=" + bread
            + " | diamond=" + diamondCount);
        health = health - 4;
        System.out.println("Injury | " + playerName
            + " | Life=" + health + " | Bread.=" + bread
            + " | diamond=" + diamondCount);
        health = health + 3;
        bread = bread - 1;
        diamondCount = diamondCount + 3;
        System.out.println("Replenishment and mining | " + playerName
            + " | Life=" + health + " | Bread.=" + bread
            + " | diamond=" + diamondCount);
    }
}

Output

Birth | Alex | Life=20 | Bread.=3 | diamond=0
Injury | Alex | Life=16 | Bread.=3 | diamond=0
Replenishment and mining | Alex | Life=19 | Bread.=2 | diamond=3

10 / 10

Continue practicing with predictions

Pet wolves can also have their own status records: name, life and number of items.

The ability map of the second lesson

StatementType and name
InitializationFirst assignment
ReadCurrent value
UpdateCalculate and write back
ExplanationCompare before and after snapshots

Select a task

Basics: Declare the quantities of five items and output a backpack list.
Advanced: Start with 0 iron ingots, get 8, use up 3, and get 2 more; the three outputs should be 8, 5, 7.
Creation: Design the pet wolf status bar to simulate feeding and consuming bones.

School selection: two boxes exchange contents

Snippet: place inside main and run separately

int leftChest = 12, rightChest = 30;
int temporary = leftChest;
leftChest = rightChest;
rightChest = temporary;
System.out.println(leftChest + " / " + rightChest);

Output

30 / 12

temporary Keep the original 12 first. The next lesson will introduce us to more types and explain why integers, decimals, true and false, and literals cannot be mixed randomly.

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