الدمج والتعارضات

Written Explanation

The basic rule: merge into the branch you are on

git switch main
git merge add-interest

The first command makes main the current branch. The second brings the commits of add-interest into it. Always switch to the branch that should receive the work first. add-interest itself is not changed.

Two kinds of merge

1. Fast-forward. If main has not moved since you created the branch, the branch is simply "ahead" of main. Git just slides the main pointer forward. No new commit is created.

Updating 728269d..f78efde
Fast-forward
 Hello.java | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

2. A merge with a merge commit. If both branches have new commits (they have diverged, as in lesson L04), Git combines both lines of work and records a merge commit, which has two parents. Git compares the two tips with their last common ancestor (a "three-way merge") and joins the changes.

Merge made by the 'ort' strategy.
 B.java | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 B.java

*   ed8f94e Merge branch 'feature'
|\  
| * 4b9cb47 Add B on feature
* | 38db0ef Edit A on main
|/  
* 42d209f Add A

The top commit is the merge commit, and its two lines join below it. Here the branches changed different files, so Git needed no help. It also works when two branches changed different parts of the same file. If your terminal opens a text editor to confirm a merge message, accept the default, save, and close it; this depends on your setup.

When Git cannot decide: a conflict

Suppose Account.java has private double rate = 0.01;. On branch add-interest you commit a change to 0.03. On main you commit a change to 0.02. Both changed the same line. Now:

git switch main
git merge add-interest

Auto-merging Account.java
CONFLICT (content): Merge conflict in Account.java
Automatic merge failed; fix conflicts and then commit the result.

The merge is now paused halfway. Nothing is broken and nothing is lost. git status explains where you are:

On branch main
You have unmerged paths.
  (fix conflicts and run "git commit")
  (use "git merge --abort" to abort the merge)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   Account.java

Reading the conflict markers

Open Account.java. Git has written both versions into the file:

public class Account {
<<<<<<< HEAD
    private double rate = 0.02;
=======
    private double rate = 0.03;
>>>>>>> add-interest
}

• <<<<<<< HEAD starts the conflict. Everything between it and ======= is your current branch (here main).
• ======= is the divider.
• Everything between ======= and >>>>>>> add-interest is the branch being merged in.
• >>>>>>> add-interest ends the conflict.

These three lines are text Git inserted. They are not code. A file can contain several conflict blocks; resolve each one.

Resolving a conflict by hand

1. Decide the correct final text. Keep one side, keep the other, combine them, or write something new. Git cannot know; you (and perhaps your teammate) decide. Suppose the team agrees on 3 percent.
2. Edit the file so it contains exactly that final text, with all three marker lines removed:

public class Account {
    private double rate = 0.03;
}

1. Stage the file to tell Git the conflict is resolved:

git add Account.java

1. Commit to finish the merge:

git commit

Git proposes a message such as Merge branch 'add-interest'. You can accept it or add a short line on how you resolved the conflict. (git commit -m "Merge branch 'add-interest'" also works.) Run git log --oneline --graph to see the merge commit with its two parents.

Before step 3, compile and test. A resolved file must be valid Java.

Changing your mind: git merge --abort

If you start a merge and decide it is the wrong moment, run:

git merge --abort

Git returns to the state before the merge. git status is clean again. Use it freely when you are unsure.

Why leftover markers break the build

Git does not check that you removed the markers. It will happily commit a file that still contains them. But the markers are not valid Java. Compiling such a file gives errors like:

Account.java:2: error: illegal start of type
<<<<<<< HEAD
^

So: after resolving, search the file for <<<<<<<, =======, and >>>>>>>, then compile before you commit.

Reducing conflicts

Conflicts are normal, but you can make them rarer and smaller: keep branches short, merge main into your branch regularly, make small focused commits, and talk to teammates about who edits which file. IDEs (for example IntelliJ IDEA) offer a merge tool that shows the two sides next to each other; the menus differ by IDE and version. Doing it by hand first shows you what those tools do.