implemented some basic getBytes methods to create bytecode for constantpool

src/main/java/nl/sander/beejava/constantpool/ConstantPool.java

@@ -1,6 +1,7 @@
 package nl.sander.beejava.constantpool;
 
 import nl.sander.beejava.constantpool.entry.NodeConstant;
+import nl.sander.beejava.util.ByteBuf;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -22,6 +23,8 @@ public class ConstantPool {
     }
 
     public byte[] getBytes() {
-        return new byte[]{};
+        ByteBuf bytes = new ByteBuf();
+        entries.forEach(entry -> bytes.add(entry.getBytes()));
+        return bytes.toBytes();
     }
 }

src/main/java/nl/sander/beejava/constantpool/entry/ConstantClass.java

@@ -1,6 +1,7 @@
 package nl.sander.beejava.constantpool.entry;
 
 public class ConstantClass extends NodeConstant {
+    private static final byte TAG = 7;
     private final ConstantUtf8 name;
 
     public ConstantClass(ConstantUtf8 name) {
@@ -20,7 +21,7 @@ public class ConstantClass extends NodeConstant {
     }
 
     @Override
-    public int getTag() {
-        return 7;
+    public byte[] getBytes() {
+        return new byte[]{TAG, upperFraction(getNameIndex()), lowerFraction(getNameIndex())};
     }
 }

src/main/java/nl/sander/beejava/constantpool/entry/ConstantDouble.java

@@ -1,6 +1,7 @@
 package nl.sander.beejava.constantpool.entry;
 
 public class ConstantDouble extends LeafConstant {
+    private static final byte TAG = 6;
     private final double doubleVal;
 
     public ConstantDouble(double doubleVal) {
@@ -8,7 +9,10 @@ public class ConstantDouble extends LeafConstant {
     }
 
     @Override
-    public int getTag() {
-        return 6;
+    public byte[] getBytes() {
+        long bits = Double.doubleToRawLongBits(doubleVal);
+        return new byte[]{TAG, rshift(bits, 56), rshift(bits, 48), rshift(bits, 40), rshift(bits, 32),
+                rshift(bits, 24), rshift(bits, 16), rshift(bits, 8), (byte) (bits & 0xFF)};
     }
+
 }

src/main/java/nl/sander/beejava/constantpool/entry/ConstantDynamic.java

@@ -1,6 +1,7 @@
 package nl.sander.beejava.constantpool.entry;
 
 public class ConstantDynamic extends NodeConstant {
+    private static final byte TAG = 17;
     private final int bootstrapMethodIndex; // TODO
     private final ConstantNameAndType nameAndType;
 
@@ -10,8 +11,8 @@ public class ConstantDynamic extends NodeConstant {
     }
 
     @Override
-    public int getTag() {
-        return 17;
+    public byte[] getBytes() {
+        return new byte[]{TAG};
     }
 
     public int getNameAndTypeIndex() {

src/main/java/nl/sander/beejava/constantpool/entry/ConstantFieldRef.java

@@ -1,6 +1,8 @@
 package nl.sander.beejava.constantpool.entry;
 
 public class ConstantFieldRef extends NodeConstant {
+    private static final byte TAG = 9;
+
     private final ConstantClass constantClass;
     private final ConstantNameAndType constantNameAndType;
 
@@ -27,7 +29,8 @@ public class ConstantFieldRef extends NodeConstant {
     }
 
     @Override
-    public int getTag() {
-        return 9;
+    public byte[] getBytes() {
+        return new byte[]{TAG};
     }
+
 }

src/main/java/nl/sander/beejava/constantpool/entry/ConstantFloat.java

@@ -1,6 +1,8 @@
 package nl.sander.beejava.constantpool.entry;
 
 public class ConstantFloat extends LeafConstant {
+    private static final byte TAG = 4;
+
     private final float floatVal;
 
     public ConstantFloat(float floatVal) {
@@ -15,7 +17,8 @@ public class ConstantFloat extends LeafConstant {
     }
 
     @Override
-    public int getTag() {
-        return 4;
+    public byte[] getBytes() {
+        long bits = Float.floatToRawIntBits(floatVal);
+        return new byte[]{TAG, (byte) (bits >>> 24), (byte) (bits >>> 16), (byte) (bits >>> 8), (byte) (bits & 0xFF)};
     }
 }

src/main/java/nl/sander/beejava/constantpool/entry/ConstantInteger.java

@@ -1,6 +1,8 @@
 package nl.sander.beejava.constantpool.entry;
 
 public class ConstantInteger extends LeafConstant {
+    private static final byte TAG = 3;
+
     private final int intVal;
 
     public ConstantInteger(int integer) {
@@ -15,7 +17,7 @@ public class ConstantInteger extends LeafConstant {
     }
 
     @Override
-    public int getTag() {
-        return 3;
+    public byte[] getBytes() {
+        return new byte[]{TAG, (byte) (intVal >>> 24), (byte) (intVal >>> 16), (byte) (intVal >>> 8), (byte) (intVal & 0xFF)};
     }
 }

src/main/java/nl/sander/beejava/constantpool/entry/ConstantInterfaceMethodRef.java

@@ -1,6 +1,8 @@
 package nl.sander.beejava.constantpool.entry;
 
 public class ConstantInterfaceMethodRef extends NodeConstant {
+    private static final byte TAG = 11;
+
     private final ConstantClass constantClass;
     private final ConstantNameAndType constantNameAndType;
 
@@ -18,8 +20,9 @@ public class ConstantInterfaceMethodRef extends NodeConstant {
         return constantNameAndType.getIndex();
     }
 
+
     @Override
-    public int getTag() {
-        return 11;
+    public byte[] getBytes() {
+        return new byte[]{TAG};
     }
 }

src/main/java/nl/sander/beejava/constantpool/entry/ConstantInvokeDynamic.java

@@ -1,6 +1,8 @@
 package nl.sander.beejava.constantpool.entry;
 
 public class ConstantInvokeDynamic extends NodeConstant {
+    private static final byte TAG = 18;
+
     private final int bootstrapMethodAttrIndex; //??
     private final ConstantNameAndType constantNameAndType;
 
@@ -18,9 +20,9 @@ public class ConstantInvokeDynamic extends NodeConstant {
                 '}';
     }
 
-    @Override
-    public int getTag() {
-        return 18;
+
+    public byte[] getBytes() {
+        return new byte[]{TAG};
     }
 }
 //TODO implement later

src/main/java/nl/sander/beejava/constantpool/entry/ConstantLong.java

@@ -2,6 +2,7 @@ package nl.sander.beejava.constantpool.entry;
 
 public class ConstantLong extends LeafConstant {
 
+    private static final byte TAG = 5;
     private final long longVal;
 
     public ConstantLong(long longVal) {
@@ -16,7 +17,10 @@ public class ConstantLong extends LeafConstant {
     }
 
     @Override
-    public int getTag() {
-        return 5;
+    public byte[] getBytes() {
+        return new byte[]{TAG, rshift(longVal, 56), rshift(longVal, 48), rshift(longVal, 40), rshift(longVal, 32),
+                rshift(longVal, 24), rshift(longVal, 16), rshift(longVal, 8), (byte) (longVal & 0xFF)};
     }
+
+
 }

src/main/java/nl/sander/beejava/constantpool/entry/ConstantMethodHandle.java

@@ -2,6 +2,8 @@ package nl.sander.beejava.constantpool.entry;
 
 //TODO implement later
 public class ConstantMethodHandle extends NodeConstant {
+    private static final byte TAG = 15;
+
     private final int referenceKind;
 
     // only 1 of these can be present:
@@ -25,8 +27,7 @@ public class ConstantMethodHandle extends NodeConstant {
         return 0; //TODO implement
     }
 
-    @Override
-    public int getTag() {
-        return 15;
+    public byte[] getBytes() {
+        return new byte[]{TAG};
     }
 }

src/main/java/nl/sander/beejava/constantpool/entry/ConstantMethodRef.java

@@ -1,6 +1,8 @@
 package nl.sander.beejava.constantpool.entry;
 
 public class ConstantMethodRef extends NodeConstant {
+    private static final byte TAG = 10;
+
     private final ConstantClass constantClass;
     private final ConstantNameAndType constantNameAndType;
 
@@ -18,8 +20,8 @@ public class ConstantMethodRef extends NodeConstant {
         return constantNameAndType.getIndex();
     }
 
-    @Override
-    public int getTag() {
-        return 10;
+
+    public byte[] getBytes() {
+        return new byte[]{TAG};
     }
 }

src/main/java/nl/sander/beejava/constantpool/entry/ConstantMethodType.java

@@ -1,6 +1,8 @@
 package nl.sander.beejava.constantpool.entry;
 
 public class ConstantMethodType extends NodeConstant {
+    private static final byte TAG = 16;
+
     private final ConstantUtf8 methodDescriptor;
 
     public ConstantMethodType(ConstantUtf8 methodDescriptor) {
@@ -14,8 +16,8 @@ public class ConstantMethodType extends NodeConstant {
                 '}';
     }
 
-    @Override
-    public int getTag() {
-        return 16;
+
+    public byte[] getBytes() {
+        return new byte[]{TAG};
     }
 }

src/main/java/nl/sander/beejava/constantpool/entry/ConstantModule.java

@@ -2,6 +2,8 @@ package nl.sander.beejava.constantpool.entry;
 
 public class ConstantModule extends NodeConstant {
 
+    private static final byte TAG = 19;
+
     private final ConstantUtf8 nameEntry;
 
     public ConstantModule(ConstantUtf8 nameEntry) {
@@ -9,8 +11,11 @@ public class ConstantModule extends NodeConstant {
         this.nameEntry = nameEntry;
     }
 
-    @Override
-    public int getTag() {
-        return 19;
+    public int getNameIndex() {
+        return nameEntry.getIndex();
+    }
+
+    public byte[] getBytes() {
+        return new byte[]{TAG, upperFraction(getNameIndex()), lowerFraction(getNameIndex())};
     }
 }

src/main/java/nl/sander/beejava/constantpool/entry/ConstantNameAndType.java

@@ -1,13 +1,14 @@
 package nl.sander.beejava.constantpool.entry;
 
 public class ConstantNameAndType extends NodeConstant {
+    private static final byte TAG = 12;
     private final ConstantUtf8 name;
-    private final ConstantUtf8 type;
+    private final ConstantUtf8 descriptor;
 
-    public ConstantNameAndType(ConstantUtf8 name, ConstantUtf8 type) {
-        super(name,type);
+    public ConstantNameAndType(ConstantUtf8 name, ConstantUtf8 descriptor) {
+        super(name, descriptor);
         this.name = name;
-        this.type = type;
+        this.descriptor = descriptor;
     }
 
 
@@ -15,20 +16,19 @@ public class ConstantNameAndType extends NodeConstant {
         return name.getIndex();
     }
 
-    public int getTypeIndex() {
-        return type.getIndex();
+    public int getDescriptorIndex() {
+        return descriptor.getIndex();
     }
 
     @Override
     public String toString() {
         return "NameAndTypeEntry{" +
                 "nameIndex=" + getNameIndex() +
-                ", typeIndex=" + getTypeIndex() +
+                ", typeIndex=" + getDescriptorIndex() +
                 '}';
     }
 
-    @Override
-    public int getTag() {
-        return 12;
+    public byte[] getBytes() {
+        return new byte[]{TAG, upperFraction(getNameIndex()), lowerFraction(getNameIndex()), upperFraction(getDescriptorIndex()), lowerFraction(getDescriptorIndex())};
     }
 }

src/main/java/nl/sander/beejava/constantpool/entry/ConstantPackage.java

@@ -1,6 +1,7 @@
 package nl.sander.beejava.constantpool.entry;
 
 public class ConstantPackage extends NodeConstant {
+    private static final byte TAG = 20;
     private final ConstantUtf8 name;
 
     public ConstantPackage(ConstantUtf8 name) {
@@ -12,8 +13,7 @@ public class ConstantPackage extends NodeConstant {
         return name.getIndex();
     }
 
-    @Override
-    public int getTag() {
-        return 20;
+    public byte[] getBytes() {
+        return new byte[]{TAG, upperFraction(getNameIndex()), lowerFraction(getNameIndex())};
     }
 }

src/main/java/nl/sander/beejava/constantpool/entry/ConstantString.java

@@ -1,6 +1,7 @@
 package nl.sander.beejava.constantpool.entry;
 
 public class ConstantString extends NodeConstant {
+    private static final byte TAG = 8;
     private final ConstantUtf8 utf8;
 
     public ConstantString(ConstantUtf8 utf8) {
@@ -18,8 +19,7 @@ public class ConstantString extends NodeConstant {
                 '}';
     }
 
-    @Override
-    public int getTag() {
-        return 8;
+    public byte[] getBytes() {
+        return new byte[]{TAG, upperFraction(getUtf8Index()), lowerFraction(getUtf8Index())};
     }
 }

src/main/java/nl/sander/beejava/constantpool/entry/ConstantUtf8.java

@@ -1,6 +1,9 @@
 package nl.sander.beejava.constantpool.entry;
 
+import java.nio.charset.StandardCharsets;
+
 public class ConstantUtf8 extends LeafConstant {
+    private static final byte TAG = 1;
     private final String value;
 
     public ConstantUtf8(String utf8) {
@@ -18,8 +21,13 @@ public class ConstantUtf8 extends LeafConstant {
                 '}';
     }
 
-    @Override
-    public int getTag() {
-        return 1;
+    public byte[] getBytes() {
+        byte[] utf8Bytes = value.getBytes(StandardCharsets.UTF_8);
+        byte[] bytes = new byte[utf8Bytes.length + 3];
+        bytes[0] = TAG;
+        bytes[1] = upperFraction(bytes.length);
+        bytes[2] = lowerFraction(bytes.length);
+        System.arraycopy(utf8Bytes, 0, bytes, 3, utf8Bytes.length);
+        return bytes;
     }
 }

src/main/java/nl/sander/beejava/constantpool/entry/NodeConstant.java

@@ -14,11 +14,20 @@ public abstract class NodeConstant {
         this.children.addAll(Arrays.asList(children)); // java8 way destroys order, not desastrous, but I like to preserve it.
     }
 
-    /*
-     * The tag that indicates the entry type in the bytecode.
-     * §4.4 Table 4.4-A. Constant pool tags
+    /**
+     * return the bytes that end up in the class file.
      */
-    public abstract int getTag();
+    public abstract byte[] getBytes();
+
+    /**
+     * Convenience for test. The TAG is always the first byte of the getBytes array
+     * see §4.4 Table 4.4-A. Constant pool tags
+     *
+     * @return a byte indicating the type of the constant
+     */
+    byte getTag(){
+        return getBytes()[0];
+    }
 
     public int getIndex() {
         return index;
@@ -32,4 +41,27 @@ public abstract class NodeConstant {
         return children;
     }
 
+    /**
+     * returns lower 8 bits as byte
+     *
+     * @param u16 is assumed to be 16 bits unsigned integer
+     * @return lower 8 bits as byte
+     */
+    protected byte lowerFraction(int u16) {
+        return (byte) (u16 & 0xFF);
+    }
+
+    /**
+     * returns upper 8 bits as byte
+     *
+     * @param u16 is assumed to be 16 bits unsigned integer
+     * @return upper 8 bits as byte
+     */
+    protected byte upperFraction(int u16) {
+        return (byte) (u16 << 8);
+    }
+
+    protected byte rshift(long bits, int positions) {
+        return (byte) (bits >>> positions);
+    }
 }

src/test/java/nl/sander/beejava/constantpool/entry/TagCorrectnessTest.java

@@ -10,17 +10,17 @@ import static org.junit.jupiter.api.Assertions.assertEquals;
 public class TagCorrectnessTest {
     @Test
     public void testSpec() {
-        assertEquals(7, classEntry().getTag());
-        assertEquals(9, fieldRef().getTag());
-        assertEquals(10, new ConstantMethodRef(classEntry(), nameAndType()).getTag());
-        assertEquals(11, new ConstantInterfaceMethodRef(classEntry(), nameAndType()).getTag());
-        assertEquals(8, new ConstantString(utf8()).getTag());
+        assertEquals(1, utf8().getTag());
         assertEquals(3, new ConstantInteger(0).getTag());
         assertEquals(4, new ConstantFloat(0).getTag());
         assertEquals(5, new ConstantLong(0).getTag());
         assertEquals(6, new ConstantDouble(0).getTag());
+        assertEquals(7, classEntry().getTag());
+        assertEquals(8, new ConstantString(utf8()).getTag());
+        assertEquals(9, fieldRef().getTag());
+        assertEquals(10, new ConstantMethodRef(classEntry(), nameAndType()).getTag());
+        assertEquals(11, new ConstantInterfaceMethodRef(classEntry(), nameAndType()).getTag());
         assertEquals(12, nameAndType().getTag());
-        assertEquals(1, utf8().getTag());
         assertEquals(15, new ConstantMethodHandle(0).getTag()); //TODO
         assertEquals(16, new ConstantMethodType(utf8()).getTag()); //TODO
         assertEquals(17, new ConstantDynamic(0, nameAndType()).getTag()); //TODO