The genealogy for David has gaps in it. Notice the large time spans between Hezron and Ram and Amminadab. Also notice the time spans between Salmon and Boaz. Does this mean that the Bible lies? Absolutely not. In hebrew, the word for son can also mean decendant. Thus the Bible is giving us truthful information. We just need to understand exactly what GOD is wanting to tell us. Unlike the genealogy found in Genesis, the fathers age when their son was born is not given. Does this mean that we can't trust genealogies outside of Genesis? Absolutely not. For instance in Jesus's genealogy, it would have been very very long to include every name. GOD knows every name and He did not think it necessary for us to know each specific name. But it still gives us all the info we need to track it back to King David or to Adam.

• Judah: Judah was one of Jacob's sons. Head of one of the 12 tribes of Israel.
• Perez: is Judah's son. Born to him from his daughter-in-law Tamar. Gen 38:6-24
• Hezron: is Perez's son. Gen 46:12

  Now Judah, Perez, and Hezron were part of Jacob's family that entered Egypt during the time of famine. While Joseph was ruling as 2nd in command in Egypt. Gen 46:8-27

  Lets jump past Ram for now

• Amminadab: was a descendant of Ram.
• Nahshon: was Amminadab's son. We are told in Ex 6:23 that he has a sister 'daughter of Amminadab' who marries Aaron. So this would confirm that Nahshon is a direct son of Amminadab.
• Aaron: Moses's brother. He became the first Levitical high priest. We know that Aaron was in his 80's when he came with Moses before Pharoah. So that places him with some years around the time of Amminadab.

  Back to Ram

• Ram: a descendant of Hezron. And Amminadab is a descendant of Ram.

  We know that Ram lived sometime between Hezron and Amminadab. We don't know exactly when. But we see that there is a gap(s) between Hezron and Amminadab.

• Salmon: was one of the Israelites who entered the promise land with Joshua.

  We are told in Matt 1:4 that Rahab (the prostitute that helped the spys sent by Joshua) was the mother of Boaz and Salmon the father (or descendant). We know that Rahab & Salmon are of the time of Joshua. And we know when they entered the promise land.

• Boaz: a well off land owner who takes Ruth as his wife via right of kinsman redemer.
• Obed: son of Boaz and Ruth. Ruth 4:17
• Jesse: was the father of David who became king of Israel.
• David: was king of Israel. Annointed by Samuel the prophet.

  We can see another gap here between Salmon and Boaz. We don't know if Obed was the direct father of Jesse. But we know that there is definitely a gap between Salmon and Boaz.