putInt java equivalent method in swift : I am looking for a java method putInt equivalent in swift and for java method System.arraycopy

I have tried with data in swift but got wrong result

            ByteBuffer Len = ByteBuffer.allocate(4);
            Len.putInt(mLen);                    // 0x00112233 order
            mBlock[mLen1] = Len.get(0);
mLen is an integer


what would be equivalent in swift and for another method 

         System.arraycopy(bData, 0, mBlock, 0, mLen);

its giving array out of bounds error