How to calculate A's position based on world space?

The problem comes from right the beginning when i started learning opengl, i just bypassed it, by translate and rotate several times...I think it's time to get it done in the right way.

the question is:  ( not real code, just describe my question)

for example, my body was placed in world space, my arm's position is A, my thumb 's position is B, relative to arm A, i want to know B's position relative to world space.

gl.glMatrixMode(GL2.GL_MODELVIEW);
gl.glLoadIdentity();

gl.glTranslatef(PostionA.xyz);
gl.glRotatef(PostionA.angleY, 1f, 0f, 0f);
gl.glRotatef(PostionA.angleX, 0f, 1f, 0f);

gl.glTranslatef(PostionB.xyz);
gl.glRotatef(PostionB.angleY, 1f, 0f, 0f);
gl.glRotatef(PostionB.angleX, 0f, 1f, 0f);

how do i get "Position B's xyz and x,y,z angle" , relative to world space.