diff --git a/app/src/main/java/hatelace/Instruction.java b/app/src/main/java/hatelace/Instruction.java
index 43d5bfb..f7d04be 100644
--- a/app/src/main/java/hatelace/Instruction.java
+++ b/app/src/main/java/hatelace/Instruction.java
@@ -1,8 +1,11 @@
 package hatelace;
 
-public interface Instruction {
-    void execute(Memory memory, ProgramCounter pc);
-    String opcode();
-    Object[] operands();
-    String toString();
+public abstract class Instruction {
+    public abstract void execute(Memory memory, ProgramCounter pc);
+
+    protected abstract String opcode();
+
+    protected abstract Object[] operands();
+
+    public abstract String toString();
 }
diff --git a/app/src/main/java/hatelace/Word.java b/app/src/main/java/hatelace/Word.java
index 4ac199f..db74dbc 100644
--- a/app/src/main/java/hatelace/Word.java
+++ b/app/src/main/java/hatelace/Word.java
@@ -1,11 +1,11 @@
 package hatelace;
 
-public interface Word {
-    <T> T getValue();
-    Word add(Word other);
-    Word subtract(Word other);
-    Word multiply(Word other);
-    Word divide(Word other);
-    String toString();
-    boolean equals(Object other);
+public abstract class Word {
+    public abstract <T> T getValue();
+    public abstract Word add(Word other);
+    public abstract Word subtract(Word other);
+    public abstract Word multiply(Word other);
+    public abstract Word divide(Word other);
+    public abstract String toString();
+    public abstract boolean equals(Object other);
 }
diff --git a/app/src/main/java/hatelace/instructions/Add.java b/app/src/main/java/hatelace/instructions/Add.java
index e45609c..fc5c2ab 100644
--- a/app/src/main/java/hatelace/instructions/Add.java
+++ b/app/src/main/java/hatelace/instructions/Add.java
@@ -2,7 +2,7 @@ package hatelace.instructions;
 
 import hatelace.*;
 
-public class Add implements Instruction {
+public class Add extends Instruction {
     private Address src;
     private Word imm;
     private Address dest;
@@ -19,7 +19,7 @@ public class Add implements Instruction {
         PC.incPC();
     }
 
-    public String opcode() {
+    protected String opcode() {
         return "ADD";
     }
 
@@ -27,7 +27,7 @@ public class Add implements Instruction {
         return String.format("%s [%s] %s [%s]", this.opcode(), this.src, this.imm, this.dest);
     }
 
-    public Object[] operands() {
+    protected Object[] operands() {
         return new Object[] { this.src, this.imm, this.dest };
     }
 }
diff --git a/app/src/main/java/hatelace/instructions/Copy.java b/app/src/main/java/hatelace/instructions/Copy.java
index 215b500..985ce1a 100644
--- a/app/src/main/java/hatelace/instructions/Copy.java
+++ b/app/src/main/java/hatelace/instructions/Copy.java
@@ -2,7 +2,7 @@ package hatelace.instructions;
 
 import hatelace.*;
 
-public class Copy implements Instruction {
+public class Copy extends Instruction {
     private Word imm;
     private Address dest;
 
@@ -17,11 +17,11 @@ public class Copy implements Instruction {
         PC.incPC();
     }
 
-    public String opcode() {
+    protected String opcode() {
         return "CPY";
     }
 
-    public Object[] operands() {
+    protected Object[] operands() {
         return new Object[] {this.imm, this.dest};
     }
 
diff --git a/app/src/main/java/hatelace/instructions/Halt.java b/app/src/main/java/hatelace/instructions/Halt.java
index 3fc8485..7823e35 100644
--- a/app/src/main/java/hatelace/instructions/Halt.java
+++ b/app/src/main/java/hatelace/instructions/Halt.java
@@ -2,7 +2,7 @@ package hatelace.instructions;
 
 import hatelace.*;
 
-public class Halt implements Instruction {
+public class Halt extends Instruction {
     /** Halts the CPU */
     public Halt(){};
 
@@ -10,11 +10,11 @@ public class Halt implements Instruction {
         PC.halt();
     }
 
-    public String opcode() {
+    protected String opcode() {
         return "HLT";
     }
 
-    public Object[] operands() {
+    protected Object[] operands() {
         return new Object[] {};
     }
 
diff --git a/app/src/main/java/hatelace/instructions/Jump.java b/app/src/main/java/hatelace/instructions/Jump.java
index 5761c4c..972945f 100644
--- a/app/src/main/java/hatelace/instructions/Jump.java
+++ b/app/src/main/java/hatelace/instructions/Jump.java
@@ -2,7 +2,7 @@ package hatelace.instructions;
 
 import hatelace.*;
 
-public class Jump implements Instruction {
+public class Jump extends Instruction {
     private int index;
 
     /** Unconditional jump, non-relative */
@@ -14,11 +14,11 @@ public class Jump implements Instruction {
         PC.setPC(this.index);
     }
 
-    public String opcode() {
+    protected String opcode() {
         return "JMP";
     }
 
-    public Object[] operands() {
+    protected Object[] operands() {
         return new Object[] {this.index};
     }
 
diff --git a/app/src/main/java/hatelace/instructions/JumpEq.java b/app/src/main/java/hatelace/instructions/JumpEq.java
index 9152637..f2538be 100644
--- a/app/src/main/java/hatelace/instructions/JumpEq.java
+++ b/app/src/main/java/hatelace/instructions/JumpEq.java
@@ -2,7 +2,7 @@ package hatelace.instructions;
 
 import hatelace.*;
 
-public class JumpEq implements Instruction {
+public class JumpEq extends Instruction {
     private int index;
     private Address src;
     private Word imm;
@@ -22,11 +22,11 @@ public class JumpEq implements Instruction {
         }
     }
 
-    public String opcode() {
+    protected String opcode() {
         return "JEQ";
     }
 
-    public Object[] operands() {
+    protected Object[] operands() {
         return new Object[] {this.index, this.src, this.imm};
     }
 
diff --git a/app/src/main/java/hatelace/instructions/Mul.java b/app/src/main/java/hatelace/instructions/Mul.java
index 0cf60da..cd61d81 100644
--- a/app/src/main/java/hatelace/instructions/Mul.java
+++ b/app/src/main/java/hatelace/instructions/Mul.java
@@ -2,7 +2,7 @@ package hatelace.instructions;
 
 import hatelace.*;
 
-public class Mul implements Instruction {
+public class Mul extends Instruction {
     private Address src1;
     private Address src2;
     private Address dest;
@@ -19,11 +19,11 @@ public class Mul implements Instruction {
         PC.incPC();
     }
 
-    public String opcode() {
+    protected String opcode() {
         return "MUL";
     }
 
-    public Object[] operands() {
+    protected Object[] operands() {
         return new Object[] { this.src1, this.src2, this.dest };
     }
 
diff --git a/app/src/main/java/hatelace/instructions/Print.java b/app/src/main/java/hatelace/instructions/Print.java
index f73ce0b..17e7ce6 100644
--- a/app/src/main/java/hatelace/instructions/Print.java
+++ b/app/src/main/java/hatelace/instructions/Print.java
@@ -2,7 +2,7 @@ package hatelace.instructions;
 
 import hatelace.*;
 
-public class Print implements Instruction {
+public class Print extends Instruction {
     private Address address;
 
     /** Print content of memory address */
@@ -15,11 +15,11 @@ public class Print implements Instruction {
         PC.incPC();
     }
 
-    public String opcode() {
+    protected String opcode() {
         return "PRT";
     }
 
-    public Object[] operands() {
+    protected Object[] operands() {
         return new Object[] { this.address };
     }
 
diff --git a/app/src/main/java/hatelace/memtypes/IntWord.java b/app/src/main/java/hatelace/memtypes/IntWord.java
index eea0583..75c00a8 100644
--- a/app/src/main/java/hatelace/memtypes/IntWord.java
+++ b/app/src/main/java/hatelace/memtypes/IntWord.java
@@ -2,7 +2,7 @@ package hatelace.memtypes;
 
 import hatelace.Word;
 
-public class IntWord implements Word {
+public class IntWord extends Word {
     private Integer value;
 
     public IntWord(Integer value) {
diff --git a/app/src/main/java/hatelace/memtypes/LongWord.java b/app/src/main/java/hatelace/memtypes/LongWord.java
index adb8855..f01ace2 100644
--- a/app/src/main/java/hatelace/memtypes/LongWord.java
+++ b/app/src/main/java/hatelace/memtypes/LongWord.java
@@ -2,7 +2,7 @@ package hatelace.memtypes;
 
 import hatelace.Word;
 
-public class LongWord implements Word {
+public class LongWord extends Word {
     private Long value;
 
     public LongWord(Long value) {