What are the rules for converting data types in Java?

The rules for data type conversion in Java are as follows:

1. Implicit type conversion, also known as automatic type conversion, occurs when a variable of one data type is assigned to a variable of another data type and the target data type has a larger range than the source data type. For example, assigning an int variable to a long variable.
2. Explicit type conversion: When a variable of one data type is assigned to a variable of another data type, if the target data type has a smaller range than the source data type, it is necessary to perform explicit type conversion. When using explicit type conversion, the target data type must be placed in parentheses before the variable, with the target data type written inside the parentheses. For example, converting a double variable to an int variable requires explicit type conversion.
3. Automatic promotion: When the operands involved in an operation have different data types, the lower data type will be automatically converted to the higher data type before the operation is performed. For example, when adding an int and a double, the int will be automatically promoted to a double before the addition is carried out.
4. Forced downgrading: When operands of different data types are involved in an operation and it is necessary to convert a higher data type to a lower data type, there is a risk of data loss, which necessitates forced downgrading. When using forced downgrading, parentheses must be added before the target data type and the target data type must be written inside the parentheses. For example, when adding a double type and an int type, the result needs to be forced downgraded to an int type.

It is important to note that when converting data types, there may be data loss or loss of precision, so proper handling and judgment are required.