Instance Fields

We can initialize fields by passing arguments to a constructor.

Sphere4 sphere = new Sphere4(3.2);

So far, we have been able to change the state (fields) of our objects by accessing them directly.

sphere.radius = 1.5;

Imagine we have a class with something sensitive, like an Account with a balance field.

• If any program can change the balance, we could have issues.

package com.example.encapsulation.drills;
public class Account {
  public double balance;
  public String accountId;

  public Account(String aId) {
    accountId = aId;
  }

  public Account(String aId, double initialBalance) {
    balance = initialBalance;
    accountId = aId;
  }

  public void deposit(double amount) {
    balance = balance + amount;
  }

  public void withdraw(double amount) {
    balance = balance - amount;
  }
}

• This is bad practice.

We should not design classes that allow others to "reach into" an object and change its state.

We can solve this by changing the visibility of our fields.

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