public class String2byteDemo {
     public static void main(String[] args) {
        String a ="2dsad132321321321";
        byte[] b = new byte[100];
        byte[] temp=a.getBytes();
        int length=temp.length;
        if(length<=100){
            for(int i=0;i<100;i++){
                if(i<length){
                    b[i]=temp[i];
                }else{
                    b[i]=0x00;
                }
            }
        }

        for(byte i:b){
            System.out.print(i+",");
        }
    }
}

------解决方案--------------------
	public static void main(String[] args) {
		String a="2afsabsdfgsdgcdbdfhrergfd1234fdsgsgsg";
		byte[] b=new byte[100];
		int i;
		for(i=0;i<a.length();i++){
			a.getBytes(i, i+1, b, i);
		}
		while(i<100){
			b[i]=0x00;
			i++;
		}
		System.out.println(Arrays.toString(b));
	}