Added javadoc, get- and add methods

src/main/java/no/ntnu/idatt1002/demo/Income.java

+package no.ntnu.idatt1002.demo;
+
+/**
+ * Income is for registering information about your income,
+ * and editing it.
+ * @author Andreas
+ */
+public class Income {
+    private double totalIncome;
+    private final double fixedIncome;
+
+    /**
+     * Class constructor
+     * @param fixedIncome   An income which you will always receive, for example a loan.
+     */
+    protected Income(double fixedIncome){
+        if(fixedIncome<0){
+            throw new IllegalArgumentException("You can not have a negativ income");
+        }
+        this.totalIncome = fixedIncome;
+        this.fixedIncome = fixedIncome;
+    }
+
+    /**
+     * Return the fixed income of the income.
+     * @return   The income´s fixed income.
+     */
+    public double getFixedIncome() {
+        double copyFixedIncome = fixedIncome;
+        return copyFixedIncome;
+    }
+
+    /**
+     * Return the total income.
+     * @return   The total income.
+     */
+    public double getIncome() {
+        double copyIncome = totalIncome;
+        return copyIncome;
+    }
+
+    /**
+     * Adding an amount to your total income.
+     * If the amount is negativ an IllegalArgumentException is thrown
+     * @param amount   The amount you want to add to your total income.
+     */
+    public void addIncome(double amount){
+        if(amount<0){
+            throw new IllegalArgumentException("You can not add a negative amount to income");
+        }
+        totalIncome+=amount;
+    }
+}

src/main/java/no/ntnu/idatt1002/demo/IncomeOverview.java

-package no.ntnu.idatt1002.demo;
-
-public class IncomeOverview {
-    private double income;
-    private final double fixedIncome;
-
-    public IncomeOverview(double fixedIncome){
-        this.income = fixedIncome;
-        this.fixedIncome = fixedIncome;
-    }
-}