It was long believed that there is a zero-point energy in the form of <mml:math display="inline"> <mml:mrow> <mml:mo></mml:mo> <mml:mi>ω</mml:mi> <mml:mo>/</mml:mo> <mml:mn>2</mml:mn> </mml:mrow>
</mml:math> for massive particles, which is obtained from Schrödinger equation for the harmonic oscillator model. In this paper, it is shown, by the Dirac oscillator, that there is no such a zero-point energy. It is argued that when a particle's wave function can spread in the whole
space, it can be static. This does neither violate wave-particle duality nor uncertainty relationship. Dirac equation correctly describes physical reality, while Schrödinger equation does not when it is not the nonrelativistic approximation of Dirac equation with a certain model. The
conclusion that there is no zero-point energy in the form of <mml:math display="inline"> <mml:mrow> <mml:mo></mml:mo> <mml:mi>ω</mml:mi> <mml:mo>/</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:math> is applied to
solve the famous cosmological constant problem for massive particles.